Methods and apparatus are taught for selectively oxidizing carbon monoxide in a source of gas containing carbon monoxide and hydrogen. A gas containing carbon monoxide and hydrogen is fed into a membrane reactor (10, 50, 60) capable of selectively absorbing the carbon monoxide. Preferably, the reactor comprises a substantially defect-free zeolite membrane (4) having at one metal that acts as an oxidation catalyst. The zeolite membrane (4) may be supported on a porous ceramic support (2, 52, 61) and the average pore diameter is preferably between about 0.3 nm and about 1.0 nm. Moreover, the substantially defect-free zeolite membrane (4) preferably has a thickness between about 0.1 micron and about 50.0 microns. The at least one metal is preferably capable of selectively oxidizing the carbon monoxide and is preferably platinum. Preferably, the temperature of reactor housing is maintained at about 200-300.degree. C.