The selective CO oxidation reaction on Pt/gamma-Al2O3 in simulated reformer gas (75% H-2; the rest is N-2) was investigated over a wide range of CO concentrations (0.02-1.5%) at low stoichiometric O-2 excess (p(O2)/p(CO) = 0.5-1.5). Integral how measurements in a microreactor showed that the optimum temperature for the PROX process (preferential oxidation) was similar to 200 degrees C; rates of CO methanation in this range were insignificant. At higher temperatures a significant loss in selectivity was observed. The quantitative determination of CO oxidation rates and selectivities as a function of CO and O-2 concentration between 150 and 250 degrees C by differential flow measurements led to reaction orders of -0.4 for p(CO) and +0.8 for P-O2 at an apparent activation energy of 71 kJ/mol. These kinetics are consistent with the selective CO oxidation reaction occurring in the low-rate branch, on a surface predominantly covered with adsorbed CO. Its ramifications on the observed dependence of the selectivity on both CO concentration and temperature are discussed. A H-2-induced increase in the CO oxidation rate by a factor of similar to 2 was observed. Comparison of the kinetic rate expression for the selective CO oxidation with the performance of a plug-flow reactor (variable flow rates) led to quantitative agreement.