diff --git a/asio/include/asio/detail/dev_poll_reactor.hpp b/asio/include/asio/detail/dev_poll_reactor.hpp
index 6bbf1b20..58deb721 100644
--- a/asio/include/asio/detail/dev_poll_reactor.hpp
+++ b/asio/include/asio/detail/dev_poll_reactor.hpp
@@ -45,7 +45,7 @@ class dev_poll_reactor
   : public asio::detail::service_base<dev_poll_reactor>
 {
 public:
-  enum { read_op = 0, write_op = 1,
+  enum op_types { read_op = 0, write_op = 1,
     connect_op = 1, except_op = 2, max_ops = 3 };
 
   // Per-descriptor data.
diff --git a/asio/include/asio/detail/epoll_reactor.hpp b/asio/include/asio/detail/epoll_reactor.hpp
index a085766b..caa7b24d 100644
--- a/asio/include/asio/detail/epoll_reactor.hpp
+++ b/asio/include/asio/detail/epoll_reactor.hpp
@@ -41,7 +41,7 @@ class epoll_reactor
   : public asio::detail::service_base<epoll_reactor>
 {
 public:
-  enum { read_op = 0, write_op = 1,
+  enum op_types { read_op = 0, write_op = 1,
     connect_op = 1, except_op = 2, max_ops = 3 };
 
   // Per-descriptor queues.
diff --git a/asio/include/asio/detail/select_reactor.hpp b/asio/include/asio/detail/select_reactor.hpp
index 161305f8..ccee8aeb 100644
--- a/asio/include/asio/detail/select_reactor.hpp
+++ b/asio/include/asio/detail/select_reactor.hpp
@@ -50,10 +50,10 @@ class select_reactor
 {
 public:
 #if defined(BOOST_WINDOWS) || defined(__CYGWIN__)
-  enum { read_op = 0, write_op = 1, except_op = 2,
+  enum op_types { read_op = 0, write_op = 1, except_op = 2,
     max_select_ops = 3, connect_op = 3, max_ops = 4 };
 #else // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
-  enum { read_op = 0, write_op = 1, except_op = 2,
+  enum op_types { read_op = 0, write_op = 1, except_op = 2,
     max_select_ops = 3, connect_op = 1, max_ops = 3 };
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)