diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index d1112b8..4d6e68d 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -13,8 +13,13 @@ module Oga
       end
 
       def evaluate(string)
-        ast     = Parser.new(string).parse
-        context = @document.children
+        ast = Parser.new(string).parse
+
+        if @document.is_a?(XML::Document)
+          context = @document.children
+        else
+          context = XML::NodeSet.new([@document])
+        end
 
         return process(ast, context)
       end
@@ -51,25 +56,12 @@ module Oga
       end
 
       def on_test(node, context)
-        ns, name = *node
-        nodes    = XML::NodeSet.new
+        nodes = XML::NodeSet.new
 
         context.each do |xml_node|
-          next unless xml_node.is_a?(XML::Element)
-
-          name_matches = xml_node.name == name || name == '*'
-          ns_matches   = false
-
-          if ns
-            ns_matches = xml_node.namespace == ns || ns == '*'
-
-          # If there's no namespace given but the name matches we'll also mark
-          # the namespace as matching.
-          elsif name_matches
-            ns_matches = true
-          end
-
-          if name_matches and ns_matches
+          # TODO: change this when attribute tests are implemented since those
+          # are not XML::Element instances.
+          if xml_node.is_a?(XML::Element) and node_matches?(xml_node, node)
             nodes << xml_node
           end
         end
@@ -77,6 +69,29 @@ module Oga
         return nodes
       end
 
+      def on_axis(node, context)
+        name, test = *node
+
+        return send("on_axis_#{name}", test, context)
+      end
+
+      def on_axis_ancestor(node, context)
+        nodes = XML::NodeSet.new
+
+        context.each do |xml_node|
+          while xml_node.respond_to?(:parent) and xml_node.parent
+            xml_node = xml_node.parent
+
+            if node_matches?(xml_node, node)
+              nodes << xml_node
+              break
+            end
+          end
+        end
+
+        return nodes
+      end
+
       def child_nodes(nodes)
         children = XML::NodeSet.new
 
@@ -88,6 +103,24 @@ module Oga
 
         return children
       end
+
+      def node_matches?(xml_node, ast_node)
+        ns, name = *ast_node
+
+        name_matches = xml_node.name == name || name == '*'
+        ns_matches   = false
+
+        if ns
+          ns_matches = xml_node.namespace == ns || ns == '*'
+
+        # If there's no namespace given but the name matches we'll also mark
+        # the namespace as matching.
+        elsif name_matches
+          ns_matches = true
+        end
+
+        return name_matches && ns_matches
+      end
     end # Evaluator
   end # XPath
 end # Oga