According to the present invention, a temperature profile within a preferential oxidation reactor is controlled using a two phase water/steam system to provide a temperature range within the reactor (10) which favors the selective oxidation of CO in a hydrogen rich reformate stream. The reformate is flowed in a mixture with oxygen over a preferential oxidation catalyst (17). The temperature profile is controlled by flowing a stream of water proximate to the preferential oxidation catalyst (17) so as the stream of water and the reformate stream passing over the catalyst (17) are in a heat transfer arrangement. The stream of water is maintained as a two phase stream from a point at which the water reaches its boiling temperature to a point proximate an outlet from which the stream of water exits the reactor (10).