This paper presents an experimental study on heat driven pumps. Two pumps with different configurations were tested, one had a simple U-tube with two check valves, while the other had an additional horizontal branch across the heating section. The dynamics of the pumps were observed in terms of pulsating pressures and temperatures inside the pumps. The pulsating cycle period and the pumping flow rate were measured at various heating powers and the inlet liquid temperatures, and the performance of the pumps were characterized. The thermal hydraulic characteristics of the pulsating cycle at different stages was proposed and discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.