The CO/CO2 product ratios at or near the surface of 9 x 10(-3) m petroleum coke spheres burning in incipiently fluidized beds of 1.82 x 10(-4), 5.95 x 10(-4), and 1.1 x 10(-3) m sand particles have been measured. The temperature of the burning particle was measured simultaneously using an embedded thermocouple. The measurements show two extrema in the CO/CO2 product ratio as the temperature of the burning particle increases from about 850 to 1650 K. A simplified homogeneous film model combustion has been developed. This model is found to predict, without any adjustable parameters, the influence of homogeneous oxidation on CO/CO2 product ratios measured by Tognotti et al. (1990, Proceedings of the 23rd Internationl Symposium on Combustion, The Combustion Institute, pp. 1207-1213). However, this model, and other boundary layer combustion models, cannot explain the experimental observations in incipiently fluidized beds which appear to reflect combustion (both homogeneous and heterogeneous) within the coke particle at lower temperatures and negligible homogeneous combustion in the boundary layer.