The paper reports on measurements of heat transfer between the walls of a cyclone and a gas-solid suspension. It is shown that the presence of the solids does not greatly enhance the total amount of heat transferred from the wall when compared with rates achieved with gas alone. It is shown that for both metallic (conducting) and non-metallic particles high rates of heat transfer are achievable between the wall and the solids, but at the same time the net heat exchange with the gas is greatly reduced. The regular, known flow pattern of solids within the cyclone renders the heat transfer process amenable to mathematical analysis. The experimental results are shown to be in reasonable agreement with predictions based on a modification of a mathematical model developed by Botterill for fluidised beds.