A two-phase flow thermosyphon solar water heating system is modelled theoretically. In the model, non-homogenous two-phase flow and heat transfer in the collector and condenser are considered, and variation of the properties of the working fluid and water with temperature are taken into account. Numerical calculations are carried out for the refrigerant R-11. The temperatures in the system, the heating and boiling section of the collector, the recirculating mass flow rate of the working fluid and the efficiency of the heating system are presented. The influences of solar insolation, water temperature and the collector tube on the efficiency of the system and on the recirculating mass flow rate are discussed.