The purpose of this article is to analyze in detail the homogeneous chemistry involving the CO oxidation in the gas around a burning char particle. Namely, the model presented in a previous work (Gonzalo-Tirado et al., 2014) [1] has been applied to the case of a 120 mu m and a 600 mu m subbituminous char particle in a 24% O-2, 1673 K atmosphere under both conventional and oxy-fuel combustion conditions. The CO + OH <-> CO2 + H reaction is shown to be the prevailing reaction in the conversion of the CO in the boundary layer; the high CO2 concentrations typical of oxy-combustion affect the equilibrium in this reaction and reduce its overall. rate, which explains the lower 'intensity' of the flame in those conditions. As for the release/absorption of heat in the gas, the reactions in which the OH radicals participate as reactants or products are predominant; the OH chemistry is somehow more intense in N-2 and higher flame temperatures and OH concentrations are thus attained in conventional combustion conditions. Relatively low moisture concentrations in the bulk gas are sufficient to activate this boundary layer chemistry; with [H2O] larger than similar to 3% no substantial changes are observed in the CO conversion. The combustion history of the particles has been also studied. A logical sequence oxidation-gasification is observed; whereas the CO-to-CO2 oxidation occurs first contiguous to the particle, the onset of char-CO2 gasification results in a detachment of the flame from the surface and a decrease in the oxidation rate, especially for large particles. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.