In this study, the performance of a solar thermal water pump with two different working fluids such as n-pentane and ethyl ether, is reported. The theoretical comparison is made in terms of the number of cycles the pump can work per day, the amount of water that can be pumped per day and the overall efficiency of the system. It is found that the efficiency of the pump with ethyl ether is about 17% higher than that with n-pentane at a discharge head of 6 m. It is shown that ethyl ether seems to be a better choice for operating a non-conventional solar thermal water pump in terms of efficiency and economics. (C) 2000 Elsevier Science Ltd. All rights reserved.