AbstractKerosine or propane was injected near the base of a small, air‐fluidized bed of sand at 940°C. The fraction of fuel burnt within the bed was determined from a heat balance, for various particle sizes, fluidizing velocities, and bed depths.Assuming the initial formation of a train of fuel vapor bubbles, theoretical analysis indicated two stages of combustion: (1) rapid consumption of the oxygen initially between fuel vapor bubbles; (2) slow consumption of oxygen initially outside the fuel vapor region by radial diffusion of oxygen and fuel vapor, analogous to a diffusion flame.Comparison of experiment with theory gave effective radial diffusion coefficients of the same order of magnitude as the molecular diffusion coefficient. It was inferred that combustion occurs largely by a diffusion flame within the bed, with diffusion through the particulate phase being the rate‐controlling step. This explains why fuel distribution is so important in attaining efficient combus