Loughborough University have designed, constructed and evaluated a compact CO preferential oxidation (PROX) reactor for PEM fuel cell applications. The reactor design is based upon the catalyst coating of high surface area heat transfer technology. Catalyst screening studies have revealed a mixed transition metal oxide promoted platinum-ruthenium formulation to be suitable for the particular reactor application i.e. acceptable CO oxidation activity and selectivity within a temperature range of 130-200 degreesC. The CO PROX reactor design has been sized for 20 kW, PEM fuel cell applications, and is based upon 2 x 2 1 catalyst washcoated compact fin heat exchangers. The PROX reactor has being successfully integrated and commissioned with a methanol steam reformer with reductions in fuel CO concentrations of 2.7% to < 20 ppm being subsequently demonstrated.