A model is developed for CO oxidation in the boundary layer of a single char particle. The model includes char oxidation and a 56 reaction gas phase kinetic scheme which is coupled with the diffusive properties of the 12 species involved in the CO oxidation. The model is compared with the experimental data of Tognotti and co-workers. The temperature reached on ignition, the resulting CO2/CO ratio, and the effect of changing water vapor concentrations are well. described. Studies on the effect of water concentration show that significant CO oxidation at low temperatures requires a high surrounding water concentration. The presence of water vapor or hydrogen is found to be necessary to obtain a high degree of CO oxidation at low temperatures; however, there is a minimum temperature below which significant CO oxidation in the boundary layer does not occur irrespective of how high the water concentration is. In addition, CO oxidation is negligible even at a surface temperature as high as 2500 K when water and hydrogen are absent. The model has also been applied to predict the effects of changing parameters. Catalytic acceleration of the rate of carbon oxidation, for example by the addition of calcium, leads to both a high particle temperature overshoot and a significant increase in CO oxidation over the particle surface.