diff --git a/Gemfile b/Gemfile
index b2c23f0..027ce0e 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,3 +2,11 @@ source 'https://rubygems.org'
 
 # Specify your gem's dependencies in boxr.gemspec
 gemspec
+
+gem "rake"
+gem "dotenv", "~> 2.0"
+gem "rspec", "~> 3.1"
+gem "simplecov", "~> 0.9"
+gem "awesome_print", "~> 1.8"
+gem "lru_redux", "~> 0.8"
+gem "parallel", "~> 1.0"
diff --git a/README.md b/README.md
index 32ca578..398731a 100644
--- a/README.md
+++ b/README.md
@@ -258,17 +258,17 @@ restore_trashed_file(file, name: nil, parent: nil)
 ```ruby
 chunked_upload_create_session_new_file(path_to_file, parent, name: nil)
 
-chunked_upload_create_session_new_file_from_io(io, parent, name)
+chunked_upload_create_session_new_file_from_io(io, parent, name, io_size: nil)
 
 chunked_upload_create_session_new_version(path_to_file, file, name: nil)
 
-chunked_upload_create_session_new_version_from_io(io, file, name)
+chunked_upload_create_session_new_version_from_io(io, file, name, io_size: nil)
 
 chunked_upload_get_session(session_id)
 
 chunked_upload_part(path_to_file, session_id, content_range)
 
-chunked_upload_part_from_io(io, session_id, content_range)
+chunked_upload_part_from_io(io, session_id, content_range, io_size: nil, io_pos: nil)
 
 chunked_upload_list_parts(session_id, limit: nil, offset: nil)
 
diff --git a/boxr.gemspec b/boxr.gemspec
index 52cdbee..021e84f 100644
--- a/boxr.gemspec
+++ b/boxr.gemspec
@@ -20,15 +20,6 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 2.0'
 
-  spec.add_development_dependency "bundler", "~> 1.6"
-  spec.add_development_dependency "rake", "~> 10.0"
-  spec.add_development_dependency "rspec", "~> 3.1"
-  spec.add_development_dependency "simplecov", "~> 0.9"
-  spec.add_development_dependency "dotenv", "~> 0.11"
-  spec.add_development_dependency "awesome_print", "~> 1.8"
-  spec.add_development_dependency "lru_redux", "~> 0.8"
-  spec.add_development_dependency "parallel", "~> 1.0"
-
   spec.add_runtime_dependency "httpclient", "~> 2.8"
   spec.add_runtime_dependency "hashie", "~> 3.5"
   spec.add_runtime_dependency "addressable", "~> 2.3"
diff --git a/lib/boxr.rb b/lib/boxr.rb
index 605d14e..3e0382e 100644
--- a/lib/boxr.rb
+++ b/lib/boxr.rb
@@ -4,6 +4,7 @@
 require 'addressable/template'
 require 'jwt'
 require 'securerandom'
+require 'stringio'
 
 require 'boxr/version'
 require 'boxr/errors'
diff --git a/lib/boxr/chunked_uploads.rb b/lib/boxr/chunked_uploads.rb
index f2e91ba..2ad549e 100644
--- a/lib/boxr/chunked_uploads.rb
+++ b/lib/boxr/chunked_uploads.rb
@@ -9,11 +9,13 @@ def chunked_upload_create_session_new_file(path_to_file, parent, name: nil)
       end
     end
 
-    def chunked_upload_create_session_new_file_from_io(io, parent, name)
+    def chunked_upload_create_session_new_file_from_io(io, parent, name, io_size: nil)
+      io_size ||= io.size
+
       parent_id = ensure_id(parent)
 
       uri = "#{UPLOAD_URI}/files/upload_sessions"
-      body = {folder_id: parent_id, file_size: io.size, file_name: name}
+      body = {folder_id: parent_id, file_size: io_size, file_name: name}
       session_info, response = post(uri, body, content_type: "application/json")
 
       session_info
@@ -27,10 +29,12 @@ def chunked_upload_create_session_new_version(path_to_file, file, name: nil)
       end
     end
 
-    def chunked_upload_create_session_new_version_from_io(io, file, name)
+    def chunked_upload_create_session_new_version_from_io(io, file, name, io_size: nil)
+      io_size ||= io.size
+
       file_id = ensure_id(file)
       uri = "#{UPLOAD_URI}/files/#{file_id}/upload_sessions"
-      body = {file_size: io.size, file_name: name}
+      body = {file_size: io_size, file_name: name}
       session_info, response = post(uri, body, content_type: "application/json")
 
       session_info
@@ -49,14 +53,17 @@ def chunked_upload_part(path_to_file, session_id, content_range)
       end
     end
 
-    def chunked_upload_part_from_io(io, session_id, content_range)
-      io.pos = content_range.min
+    def chunked_upload_part_from_io(io, session_id, content_range, io_size: nil, io_pos: nil)
+      io_size ||= io.size
+      io_pos ||= content_range.min
+
+      io.pos = io_pos
       part_size = content_range.max - content_range.min + 1
       data = io.read(part_size)
       io.rewind
 
       digest = "sha=#{Digest::SHA1.base64digest(data)}"
-      range = "bytes #{content_range.min}-#{content_range.max}/#{io.size}"
+      range = "bytes #{content_range.min}-#{content_range.max}/#{io_size}"
 
       uri = "#{UPLOAD_URI}/files/upload_sessions/#{session_id}"
       body = data
@@ -155,25 +162,55 @@ def chunked_upload_new_version_of_file_from_io(io, file, name, n_threads: 1, con
 
     PARALLEL_GEM_REQUIREMENT = Gem::Requirement.create('~> 1.0').freeze
 
-    def chunked_upload_to_session_from_io(io, session, n_threads: 1, content_created_at: nil, content_modified_at: nil)
+    def content_ranges_for(session, io_size)
       content_ranges = []
       offset = 0
       loop do
-        limit = [offset + session.part_size, io.size].min - 1
+        limit = [offset + session.part_size, io_size].min - 1
         content_ranges << (offset..limit)
-        break if limit == io.size - 1
+        break if limit == io_size - 1
 
         offset = limit + 1
       end
 
+      content_ranges
+    end
+
+    def read_io_concurrently(io, content_range, lock)
+      part_size = content_range.max - content_range.min + 1
+      buf = String.new(capacity: part_size)
+
+      pread_supported = io.respond_to?(:pread)
+      if pread_supported
+        begin
+          io.pread(part_size, content_range.min, buf)
+        rescue SystemCallError
+          pread_supported = false
+        end
+      end
+
+      unless pread_supported
+        lock.synchronize do
+          io.pos = content_range.min
+          io.read(part_size, buf)
+        end
+      end
+
+      buf
+    end
+
+    def chunked_upload_to_session_from_io(io, session, n_threads: 1, io_size: nil, content_created_at: nil, content_modified_at: nil)
+      io_size ||= io.size
+
+      content_ranges = content_ranges_for(session, io_size)
       parts =
         if n_threads > 1
           raise BoxrError.new(boxr_message: "parallel chunked uploads requires gem parallel (#{PARALLEL_GEM_REQUIREMENT}) to be loaded") unless gem_parallel_available?
 
+          lock = Mutex.new
           Parallel.map(content_ranges, in_threads: n_threads) do |content_range|
-            File.open(io.path) do |io_dup|
-              chunked_upload_part_from_io(io_dup, session.id, content_range)
-            end
+            data = read_io_concurrently(io, content_range, lock)
+            chunked_upload_part_from_io(StringIO.new(data), session.id, content_range, io_size: io_size, io_pos: 0)
           end
         else
           content_ranges.map do |content_range|
diff --git a/spec/boxr/chunked_uploads_spec.rb b/spec/boxr/chunked_uploads_spec.rb
index 95b25ed..30715b4 100644
--- a/spec/boxr/chunked_uploads_spec.rb
+++ b/spec/boxr/chunked_uploads_spec.rb
@@ -1,6 +1,9 @@
 #rake spec SPEC_OPTS="-e \"invokes chunked uploads operations"\"
+
+require "parallel"
+
 describe "chunked uploads operations" do
-  it "invokes chunked uploads operations" do
+  it "invokes chunked uploads session-related operations" do
     puts "create chunked upload session"
     new_session = BOX_CLIENT.chunked_upload_create_session_new_file("./spec/test_files/#{TEST_LARGE_FILE_NAME}", @test_folder)
     expect(new_session.id).not_to be_nil
@@ -16,14 +19,35 @@
     puts "abort chunked upload session"
     abort_info = BOX_CLIENT.chunked_upload_abort_session(new_session.id)
     expect(abort_info).to eq({})
+  end
 
-    puts "upload a large file in chunks"
-    new_file = BOX_CLIENT.chunked_upload_file("./spec/test_files/#{TEST_LARGE_FILE_NAME}", @test_folder)
-    expect(new_file.name).to eq(TEST_LARGE_FILE_NAME)
-    test_file = new_file
+  shared_examples "chunked uploads data-upload" do |n_threads:|
+    it "uploads chunked uploads upload-related operations (threads: #{n_threads})" do
+        puts "upload a large file in chunks"
+        new_file = BOX_CLIENT.chunked_upload_file("./spec/test_files/#{TEST_LARGE_FILE_NAME}", @test_folder, n_threads: n_threads)
+        expect(new_file.name).to eq(TEST_LARGE_FILE_NAME)
+        test_file = new_file
 
-    puts "upload new version of a large file in chunks"
-    new_version = BOX_CLIENT.chunked_upload_new_version_of_file("./spec/test_files/#{TEST_LARGE_FILE_NAME}", test_file)
-    expect(new_version.id).to eq(test_file.id)
+        puts "upload new version of a large file in chunks"
+        new_version = BOX_CLIENT.chunked_upload_new_version_of_file("./spec/test_files/#{TEST_LARGE_FILE_NAME}", test_file, n_threads: n_threads)
+        expect(new_version.id).to eq(test_file.id)
+
+        puts "upload a large file in chunks from IO stream"
+        filename = "yet another large file.txt"
+        io = StringIO.new
+        io << "1" * 21 * 1024**2
+        io.rewind
+        new_file = BOX_CLIENT.chunked_upload_file_from_io(io, @test_folder, filename, n_threads: n_threads)
+        expect(new_file.name).to eq(filename)
+        test_file = new_file
+
+        puts "upload new version of a large file in chunks from IO stream"
+        io.rewind
+        new_version = BOX_CLIENT.chunked_upload_new_version_of_file_from_io(io, test_file, filename, n_threads: n_threads)
+        expect(new_version.id).to eq(test_file.id)
+    end
   end
+
+  it_behaves_like "chunked uploads data-upload", n_threads: 1
+  it_behaves_like "chunked uploads data-upload", n_threads: 2
 end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 97ab9d5..10bab73 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -4,7 +4,7 @@
 require 'awesome_print'
 
 RSpec.configure do |config|
-  config.before(:each) do
+  config.before(:each) do |test|
     if test.metadata[:skip_reset]
       puts "Skipping reset"
       next