This paper focuses on the experimental analysis of the thermal behavior of two-phase closed thermosyphons with an unusual geometry characterized by a semicircular condenser and a straight evaporator. All the tests were done in an experimental indoor setup that uses electrical skin heaters to simulate the solar radiation. Different evaporator length, fill ratio of working fluid, cooling temperature and slope of the evaporator were tested for different heat fluxes, and the effects of these parameters on the overall thermal resistance were verified. An analysis of the transient results and the steady state performance was conducted in order to provide information for the design of a compact solar domestic hot-water system. (C) 2003 Elsevier Ltd. All rights reserved.