We describe a study of the combustion of charges of coke with a wide range of particle sizes in fluidised beds of sand. The coke particles ranged in size from 2.0 to 6.5 mm and experiments were performed with small and large charges of particles. In the latter case the experiments were performed only in conditions where the mass transfer resistance of O2 from the bubbles to the particulate phase had a negligible contribution to the overall O2 mass transfer resistance. The beds of sand were made of closely sized particles with average diameters of 0.23, 0.46 and 0.65 mm. The beds were operated at atmospheric pressure and at a temperature of 1223 K. A simple theory was developed, based on the findings of Pinto and Guedes de Carvalho (1990, Trans. Instn chem. Engrs 68A, 503-509) and Guedes de Carvalho et al. (1991, Trans. Instn chem. Engrs 69A, 63-70), to predict the mass transfer resistance of O2 from the particulate phase to the burning particles and the burn-out time. This theory shows excellent agreement with the experimental data.