Application of two-phase closed thermosiphons to heat recovery systems has led the authors to investigate the performance of thermosiphons at medium temperatures. Two-phase closed thermosiphons working under various conditions have been tested and their thermal performance has been measured for mean evaporator wall temperatures between 100°C and 250°C. A description of the design and construction of the test facility is included. Aspects of safety of container materials have been investigated for water as the working fluid. It was found that copper-nickel alloys and carbon-manganese stainless steel are suitable container materials for the range of temperatures considered. The critical heat flux and dry-out limit were observed in experiments with a 13.2 mm diameter thermosiphon. Boiling heat transfer phenomena and overall thermal conductance have been experimentally investigated. It was found that an increase in thermosiphon diameter changes the boiling mechanism from saturated film boiling to nucleate boiling. The test results show a good agreement with published correlation criteria. Further experiments are needed to determine the optimum pipe diameter for the applications under consideration.