A theory is proposed to describe the combustion of coke with high ash content in fluidised beds of sand. It is shown that, during combustion, the carbon rate of consumption is well described taking the coke particle as an unreacted core surrounded by a structure of agglutinated ashes. The parameters that characterize the O-2 mass transfer rate from the particulated phase to the ash surface (Sh’/epsilon(mf)) and from the ash surface to the unreacted core (D-eff) are determined and it is shown how D-eff changes with the carbon content of the particles. The tortuosity of the diffusion path through the ash layer is obtained from the effective diffusivity, and the values found (between three and eight) are of the order of magnitude of those cited in the literature for catalyst particles. The experimental work was carried out in beds of sand 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. Individual particles with 5 mm of initial diameter were charged into the bed and their carbon content was determined by continuous analysis of the CO2 in the gases leaving the bed.