diff --git a/lib/oga/xpath/compiler.rb b/lib/oga/xpath/compiler.rb
index 12dc638..422ae70 100644
--- a/lib/oga/xpath/compiler.rb
+++ b/lib/oga/xpath/compiler.rb
@@ -447,7 +447,7 @@ module Oga
       # @return [Oga::Ruby::Node]
       #
       def on_predicate_index(test, predicate, input)
-        int1      = literal('1')
+        int1      = literal(1)
         index     = to_int(predicate)
         index_var = literal(:index)
 
@@ -732,9 +732,9 @@ module Oga
           raise TypeError, 'count() can only operate on NodeSet instances'
         end
 
-        count.assign(literal('0.0'))
+        count.assign(literal(0.0))
           .followed_by do
-            process(arg, input) { count.assign(count + literal('1')) }
+            process(arg, input) { count.assign(count + literal(1)) }
           end
           .followed_by(count)
       end
@@ -1063,6 +1063,46 @@ module Oga
           end
       end
 
+      # @param [Oga::Ruby::Node] input
+      # @param [AST::Node] haystack
+      # @param [AST::Node] start
+      # @param [AST::Node] length
+      # @return [Oga::Ruby::Node]
+      def on_call_substring(input, haystack, start, length = nil)
+        haystack_var = unique_literal(:haystack)
+        start_var    = unique_literal(:start)
+        stop_var     = unique_literal(:stop)
+        length_var   = unique_literal(:length)
+        conversion   = literal(Conversion)
+
+        haystack_var.assign(try_match_first_node(haystack, input))
+          .followed_by do
+            start_var.assign(try_match_first_node(start, input))
+              .followed_by do
+                start_var.assign(start_var - literal(1))
+              end
+          end
+          .followed_by do
+            if length
+              length_var.assign(try_match_first_node(length, input))
+                .followed_by do
+                  length_int = conversion.to_float(length_var)
+                    .to_i - literal(1)
+
+                  stop_var.assign(start_var + length_int)
+                end
+            else
+              stop_var.assign(literal(-1))
+            end
+          end
+          .followed_by do
+            substring = conversion
+              .to_string(haystack_var)[range(start_var, stop_var)]
+
+            block_given? ? substring.empty?.not.if_true { yield } : substring
+          end
+      end
+
       ##
       # Delegates type tests to specific handlers.
       #
@@ -1108,6 +1148,13 @@ module Oga
         Ruby::Node.new(:lit, [value.to_s])
       end
 
+      # @param [Oga::Ruby::Node] start
+      # @param [Oga::Ruby::Node] stop
+      # @return [Oga::Ruby::Node]
+      def range(start, stop)
+        Ruby::Node.new(:range, [start, stop])
+      end
+
       # @param [String] name
       # @return [Oga::Ruby::Node]
       def unique_literal(name)
diff --git a/spec/oga/xpath/compiler/calls/substring_spec.rb b/spec/oga/xpath/compiler/calls/substring_spec.rb
index 4f2ea62..172e465 100644
--- a/spec/oga/xpath/compiler/calls/substring_spec.rb
+++ b/spec/oga/xpath/compiler/calls/substring_spec.rb
@@ -4,26 +4,37 @@ describe Oga::XPath::Compiler do
   describe 'substring() function' do
     before do
       @document = parse('<root><a>foobar</a><b>3</b></root>')
+
+      @a1 = @document.children[0].children[0]
     end
 
-    it 'returns the substring of a string' do
-      evaluate_xpath(@document, 'substring("foo", 2)').should == 'oo'
+    describe 'at the top-level' do
+      it 'returns the substring of a string' do
+        evaluate_xpath(@document, 'substring("foo", 2)').should == 'oo'
+      end
+
+      it 'returns the substring of a string using a custom length' do
+        evaluate_xpath(@document, 'substring("foo", 2, 1)').should == 'o'
+      end
+
+      it 'returns the substring of a node set' do
+        evaluate_xpath(@document, 'substring(root/a, 2)').should == 'oobar'
+      end
+
+      it 'returns the substring of a node set with a node set as the length' do
+        evaluate_xpath(@document, 'substring(root/a, 1, root/b)').should == 'foo'
+      end
+
+      it 'returns an empty string when the source string is empty' do
+        evaluate_xpath(@document, 'substring("", 1, 3)').should == ''
+      end
     end
 
-    it 'returns the substring of a string using a custom length' do
-      evaluate_xpath(@document, 'substring("foo", 2, 1)').should == 'o'
-    end
-
-    it 'returns the substring of a node set' do
-      evaluate_xpath(@document, 'substring(root/a, 2)').should == 'oobar'
-    end
-
-    it 'returns the substring of a node set with a node set as the length' do
-      evaluate_xpath(@document, 'substring(root/a, 1, root/b)').should == 'foo'
-    end
-
-    it 'returns an empty string when the source string is empty' do
-      evaluate_xpath(@document, 'substring("", 1, 3)').should == ''
+    describe 'in a predicate' do
+      it 'returns a NodeSet containing all matching nodes' do
+        evaluate_xpath(@document, 'root/a[substring("foo", 1, 2)]')
+          .should == node_set(@a1)
+      end
     end
   end
 end