A preferential oxidation reactor (PrOx) is provided including a plurality of substrates defining a plurality of channels, through which a reformate stream flows. A CO-sorption layer and a CO-catalyst layer coat a surface of each substrate support member. The PrOx operates in a first mode, generally at a temperature below 100 .degree. C., whereby the CO-sorption material adsorbs CO from the reformate stream. After operation in the first mode, the PrOx operates in a second mode, generally at a temperature above 100.degree. C., whereby the CO-catalyst material enables a preferential oxidation reaction of CO in the reformate stream with a supply of oxygen and desorption of the CO previously absorbed by the CO-sorption layer for an overall reduction in the CO content of the reformate stream.