The principle of the workings of a solar thermal water pump with n-pentane as the working fluid is described briefly. The performance of the pump is predicted from the thermodynamic analysis of the cycle of events. Experiments were carried out with a small solar thermal pump having a collector area of 1 m(2). The experimental and theoretical results were compared. The experimental results were different from those obtained theoretically for ideal conditions. But, during the course of experimentation a number of constraints were noticed. When the theoretical analysis was appropriately modified to account for these limitations, the resulting agreement was found to be good which validates the model proposed.