Appts. for selectively oxidising CO to CO2 in a fuel stream contg. H2 and CO, comprises: (a) a prim. reaction chamber with catalyst for promoting oxidn. of CO to CO2 and having first and second inlets; (b) a second reaction chamber for promoting oxidn. of CO to CO2 in fluid communication with the prim. chamber, and having a first outlet in fluid communication with the first inlet, through at least a portion of each of the prim. and second chambers; (c) a third reaction chamber for promoting oxidn. of CO to CO2 in fluid communication with the prim. chamber, and with a second outlet in fluid communication with the second inlet through at least a portion of each of the prim. and third chambers; (d) a first valve for rendering the second inlet and second outlet impassable to fluid flow when the first inlet and first outlet are passable to fluid flow; and (e) a second valve for rendering the first inlet and the first outlet impassable to fluid flow when the second inlet and second outlet are passable to fluid flow. Also claimed is a method for controlling the rate of selective oxidn. of CO to CO2 in a fuel stream comprising: (i) measuring the fuel gas stream flow rate introduced to the reaction chamber at the inlet; (ii) actuating the oxygen contg. gas inlet valve, so that the flow rate of the gas, proportionate to the flow rate of the fuel gas is inputted; (iii) increasing the oxygen contg. gas flow rate, when the measured temp. difference between a first and second thermocouple is greater than a pre-set control value; and (iv) reducing the oxygen contg. gas flow rate, when the temp. difference between the thermocouples is below the pre-set value    USE -   Used as a method of selectively oxidising CO present in a mixt. of gases to CO2, esp. for fuel cells.    ADVANTAGE -   Allows the use of impure hydrogen gas supply for fuel cells without causing CO poisoning of the Pt catalysts used, by maintaining the CO concn. below 10 ppm. therefore reducing fuel cell operating costs considerably. Also, the use of liq. fuels is allowed, obviating the need for pressurised bottling equipment.