The present work deals with a theoretical analysis by computer simulation of a two phase thermosyphon solar domestic hot water system using R-ll as a working fluid. The performance was studied by the aid of a simulation procedure for a typical day period divided into equal time intervals of 15 min. Instantaneous values of solar radiation intensity and ambient temperature were applied at the beginning of each time step. The insolation was calculated according to the procedure suggested by ASHRAE. The variation of ambient temperature during the day was approximated by a sine function. The variation of working fluid properties with temperature was also taken into account. The computer program and calculation procedure were first validated by comparing the results with established results of single phase systems. Then, calculations were performed for the two phase thermosyphon system to evaluate mass flow rate, saturation pressure and temperature in the collector and condenser, together with tank temperature, and collector and condenser thermal efficiencies. These calculated values were obtained for three cases, namely, the no loading case of no water withdrawal from the tank, continuous loading and intermittent loading. The results obtained showed that, in the two phase system, the saturation pressure and temperature increase continuously during the day and follow the tank temperature pattern. This makes the system pressure dependent on the tank loading. The collector efficiency did not reveal a serious change with the loading condition. This behavior differs from that which occurs in the single phase system, where the collector efficiency increases considerably with loading. Comparison of the two phase system with a single phase system of the same collector area and tank volume showed a significant increase in collector efficiency and tank temperature for the case of the two phase system.