NOVELTY - A temperature profile within a preferential oxidation reactor is controlled by flowing mixture of hydrogen rich reformate and oxygen over the preferential oxidation catalyst, flowing a stream of water near the catalyst, and maintaining the stream of water as to two phase stream from a point at which the water reaches its boiling temperature to point from which the water exits the reactor.    USE - For controlling the temperature profile within a preferential oxidation reactor e.g. for fuel cell.    ADVANTAGE - The inventive method provides a temperature range within the reactor that favors the selective oxidation of carbon monoxide in a hydrogen rich reformate stream.    DETAILED DESCRIPTION - Controlling of temperature profile within a preferential oxidation reactor (10) comprises flowing stream of mixture of hydrogen rich reformate and oxygen over the preferential oxidation catalyst, flowing a stream of water near the preferential oxidation catalyst so that the stream of water and the reformate stream passing over the catalyst are in the heat transfer arrangement, and maintaining the stream of water as to two phase stream from a point at which the water reaches its boiling temperature to point near an outlet from which the stream of the water exits the reactor. INDEPENDENT CLAIMS are also included for:    (a) a carbon monoxide selective oxidation reactor comprising reactor body (11), reformate inlet (12), oxygen inlet (22), catalyst for selective oxidation of carbon monoxide, heat exchanger, and stream of water flowing through the heat exchanger; and    (b) a method of making a heat exchanger comprising forming a tubular conduit of first metal into a tubular section, placing the tubular section in a die, and casting a second metal on outer surface of the tubular section in the form of fins.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of a preferential oxidation reactor in the invention.    Preferential oxidation reactor (10)    Reactor body (11)    Reformate inlet (12)    Oxygen inlet (22)    Water outlet (38)