This paper deals with the study of heat transfer behaviour in the cyclone separator of a circulating fluidized bed (CFB) and how it is affected by relevant operating parameters. The experiments are conducted in a 140 mm diameter cyclone of a cold bed set-up of 102 mm x 102 mm bed cross-section, 5.25 m height CFB. The cyclone separator is designed according to the high-efficiency Lapple design and made to accommodate two identical heat transfer probes. From the experimental results, the heat transfer coefficient is found to increase with increase in solid circulation rate, as well as gas superficial velocity. The effect of bed inventory and heat flux on heat transfer coefficient has also been investigated. An empirical equation has been developed to predict the heat transfer coefficient in the cyclone separator based on dimensional analysis. The experimental results are compared with the predicted results and a good agreement has been observed. There is an optimal distance from the entry of the cyclone where the local heat transfer coefficient is maximum. The collection efficiency of the cyclone separator has been measured for all the operating conditions. Copyright (C) 2000 John Wiley & Sons, Ltd.