In this study, the thermal performance of a two-phase, closed thermosyphon (TPCT) was investigated experimentally. In the experiments, we used water, ethanol, and ethylene glycol as the working fluids and considered different operating conditions, such as inclination angles (30, 60, and 90 degrees), heat inputs (200, 400, and 600W), and flow rates of cooling water (10, 20, and 30 L/h). Initially, an experimental test rig was designed and built, and, then, a series of rigorous tests was conducted on the TPCT. In the experiments, the temperature distribution on the wickless heat pipe or the thermosyphon's surface and the temperature differences of the cooling water were measured. The data were used to calculate the thermal resistance and efficiency of the TPCT, and the results are presented graphically and discussed in detail. The efficiency of the TPCT for each working fluid at different operating conditions varied between averages of 30 and 95%, which was in good agreement with published results. The results indicated that water was the best working fluid when the heat inputs were 200W and the flow rates of the cooling water were 10 L/h. Ethylene glycol was the best working fluid when the heat inputs were 200W and the flow rates of the cooling water were 30 L/h. Ethanol was the best working fluid when the heat inputs were 600W and the flow rates of the cooling water were 10 L/h. In addition, it was found that the inclination angle and heat inputs had significant effects on the efficiency of the TPCT. (C) 2016 Elsevier B.V. All rights reserved.