The paper reports an experimental and theoretical investigation on heat transfer to the walls of the standpipe of a circulating fluidized bed boiler. The heat transfer was studied in a standpipe test section, 1940 mm long and 100 mm square cross section. Solids 3 used in this study were silica-alumina ceramic and sand with densities of 700 and 2564 kg/m(3), and sizes of 130 and 266 mum, respectively. The results show that there are two distinct zones: dense and dilute. For both types of particles, the heat transfer is higher in the dense section. The heat transfer coefficient increases with increase in the solids circulation rate. For a given circulation rate, a finer particle yields higher heat transfer coefficient. The paper also presents a mechanistic model of heat transfer in the standpipe. Heat transfer coefficients predicted by the proposed model agree well with the experimental data.