An apparatus and method for the preferential oxidation of carbon monoxide in a hydrogen-rich fluid. The apparatus utilizes one or more reactors that are dimensioned to optimize the exothermic oxidation reaction and the transfer of heat to and from the catalyst bed. A reactor of the apparatus has an elongated cylindrical catalyst bed and heat transfer means adjacent the catalyst bed. The heat transfer means is suitable for pre-heating the catalyst bed during start-up operations and for removing the heat from the catalyst bed during the oxidation reaction. One or more reactors of different dimensions may be utilized depending upon the pressure of the hydrogen-rich fluid to be directed into the apparatus and the pressure requirements for the carbon monoxide-depleted fluid exiting the apparatus. For instance, in low pressure operations where it may be desirable to minimize the pressure drop across the apparatus, two or more reactors having relatively smaller dimensions can be utilized. In higher pressure operations where pressure drop across the preferential oxidation apparatus is of less concern, a single reactor having larger dimensions may be utilized. The relatively narrow dimension of the catalyst bed coupled with the relatively large surface are used for heat transfer provides for more uniform temperatures within the catalyst bed and improved temperature control of the bed.