This paper investigates the heterogeneous and homogeneous reactions involved in the formation and reduction of carbon monoxide (CO) in a 100 kW oxy-fuel-fired circulating fluidized-bed furnace, using CO as an indicator of the progress of combustion. A mathematical combustion model, which takes into account both the mixing and kinetics, is developed for simulating char and gas combustion under air- and oxy-fuel-fired conditions. The experimental results demonstrate that, for similar fluid dynamic and thermal conditions, oxy-fuel conditions typically yield a vertical CO profile with higher peak concentrations than are obtained with air-firing, mainly because of the higher inlet concentrations of O-2, whereby CO released from the increased fuel feeding is diluted in the same total volumetric flow of gas. On the basis of a comparison of the modeling results and the experimental results, it is proposed that the effects of CO2 should be considered in determining the rate of CO oxidation that is valid for both air- and oxy-fuel-fired conditions. The vertical in-furnace profiles of CO from the combustion model were compared to the corresponding experimental results from five test runs under air- and oxy-fuel-fired conditions. The results show good agreement, with the agreement for oxy-fuel-fired conditions made possible by adding a CO2-dependent term to the expression for the CO oxidation rate.