The effects of superficial gas velocity (0.5 to 4.5 m/s), solid circulating rate (0 to 40 kg/m(2).s), suspension density (0 to 19 kg/m(3)) and particle sizes (83, 103, 163, 236 mu m) on the bed-to-wall heat transfer coefficient have been determined in a downer reactor (0.1 m I.D. x 3.5 m high). Bed-to-wall heat transfer coefficient increases with increasing suspension density. The heat transfer coefficient by gas convection played a significant role, especially at lower solid circulation rates or suspension densities and larger particle sizes. At a given particle suspension density in the downer reactor, the heat transfer coefficient increases with decreasing particle size. A model is proposed to predict the bed-to-wall heat transfer coefficient in a downer reactor.