diff --git a/CHANGELOG.md b/CHANGELOG.md
index 136fd41..af01b75 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@
 - Remove inline alignment from tables.
 - Render `<figure>` for images with title.
 - Render abbreviations with (e.g. `*[CSS]: Cascading Style Sheets`).
+- Ensure correct numerical sorting of TOC when there are more than 10 chapters
 
 ## v3.1.0
 
diff --git a/lib/kitabu/exporter/epub.rb b/lib/kitabu/exporter/epub.rb
index 716f5ec..e8ebba6 100644
--- a/lib/kitabu/exporter/epub.rb
+++ b/lib/kitabu/exporter/epub.rb
@@ -8,13 +8,14 @@ class Epub < Base
 
       def sections
         @sections ||=
-          html.css(SECTION_SELECTOR).each_with_index.map do |chapter, index|
+          html.css(SECTION_SELECTOR).each.with_index(1).map do |chapter, index|
             html = Nokogiri::HTML5.fragment(chapter.inner_html)
+            section_number = index.to_s.rjust(2, '0')
 
             OpenStruct.new(
               index:,
-              filename: "section_#{index}.html",
-              filepath: tmp_dir.join("section_#{index}.html").to_s,
+              filename: "section_#{section_number}.html",
+              filepath: tmp_dir.join("section_#{section_number}.html").to_s,
               html:
             )
           end
diff --git a/spec/kitabu/exporter/epub_spec.rb b/spec/kitabu/exporter/epub_spec.rb
index afa999c..073da70 100644
--- a/spec/kitabu/exporter/epub_spec.rb
+++ b/spec/kitabu/exporter/epub_spec.rb
@@ -5,10 +5,21 @@
 describe Kitabu::Exporter::Epub do
   let(:root) { SPECDIR.join("support/mybook") }
 
-  it "generates e-pub" do
+  before do
     Kitabu::Exporter::HTML.export(root)
     Kitabu::Exporter::Epub.export(root)
+  end
 
+  it "generates e-pub" do
     expect(root.join("output/mybook.epub")).to be_file
   end
+
+  it "adds leading zero to 1 digit number" do
+    sections = root.join("output/epub").glob("section*.{xhtml,html}")
+    sections = sections.map do |path|
+      path.to_s.split("/").last
+    end
+    expected = 1.upto(12).map {|i| "section_#{i.to_s.rjust(2, '0')}.html" }
+    expect(sections).to eq(expected)
+  end
 end
diff --git a/spec/support/mybook/text/05_Chapter 5.md b/spec/support/mybook/text/05_Chapter 5.md
new file mode 100644
index 0000000..aea40dd
--- /dev/null
+++ b/spec/support/mybook/text/05_Chapter 5.md	
@@ -0,0 +1,3 @@
+## Chapter 5
+
+This is Chapter 5
diff --git a/spec/support/mybook/text/06_Chapter 6.md b/spec/support/mybook/text/06_Chapter 6.md
new file mode 100644
index 0000000..2b84380
--- /dev/null
+++ b/spec/support/mybook/text/06_Chapter 6.md	
@@ -0,0 +1,3 @@
+## Chapter 6
+
+This is Chapter 6
diff --git a/spec/support/mybook/text/07_Chapter 7.md b/spec/support/mybook/text/07_Chapter 7.md
new file mode 100644
index 0000000..3bd57c8
--- /dev/null
+++ b/spec/support/mybook/text/07_Chapter 7.md	
@@ -0,0 +1,3 @@
+## Chapter 7
+
+This is Chapter 7
diff --git a/spec/support/mybook/text/08_Chapter 8.md b/spec/support/mybook/text/08_Chapter 8.md
new file mode 100644
index 0000000..9416525
--- /dev/null
+++ b/spec/support/mybook/text/08_Chapter 8.md	
@@ -0,0 +1,3 @@
+## Chapter 8
+
+This is Chapter 8
diff --git a/spec/support/mybook/text/09_Chapter 9.md b/spec/support/mybook/text/09_Chapter 9.md
new file mode 100644
index 0000000..01b85f7
--- /dev/null
+++ b/spec/support/mybook/text/09_Chapter 9.md	
@@ -0,0 +1,3 @@
+## Chapter 9
+
+This is Chapter 9
diff --git a/spec/support/mybook/text/10_Chapter 10.md b/spec/support/mybook/text/10_Chapter 10.md
new file mode 100644
index 0000000..44415aa
--- /dev/null
+++ b/spec/support/mybook/text/10_Chapter 10.md	
@@ -0,0 +1,3 @@
+## Chapter 10
+
+This is Chapter 10
diff --git a/spec/support/mybook/text/11_Chapter 11.md b/spec/support/mybook/text/11_Chapter 11.md
new file mode 100644
index 0000000..78fd39b
--- /dev/null
+++ b/spec/support/mybook/text/11_Chapter 11.md	
@@ -0,0 +1,3 @@
+## Chapter 11
+
+This is Chapter 11