One important issue in fluidized bed combustion is the effect of the fluidized particles on the homogeneous oxidation of both the volatiles and CO from char combustion. These surfaces are thought to quench the radicals and thus lower the rate of combustion significantly. In this work the quenching of the radicals is introduced in a detailed chemical kinetic mechanism to model the effect of the solids on the oxidation of CO in the particulate and bubble phases. In this way the effect of particle concentration, particle diameter, temperature, air-to-fuel ratio, and water concentration has been studied. The results do confirm that oxidation rates are significantly reduced in the particulate phase, although Do complete suppression was found. In the bubble phase the effect of heterogeneous radical quenching following mass transfer into the particulate phase and also quenching on the solids at a bubble's surface is small. No critical (minimum) bubble diameter for the ignition of CO in the bubble phase was found under the investigated conditions. A comparison with the literature on the oxidation of CH4 in an incipiently fluidized bed confirms the predictions for the oxidation of CO.