Experimental investigations were carried out to study heat transfer characteristics of titanium (commercially pure titanium, TA2)/water two-phase closed thermosyphon (Ti/H2O TPCT). Experiments of copper/water (Cu/H2O) TPCT with same dimension and manufacturing process had also been performed for contrast. Experimental results show that there are no remarkable differences of heat transfer coefficients in evaporator (h(c)) between the two kinds of TPCTs, whereas surprisingly the experimental results of heat transfer coefficient in condenser (h(c)) of Ti/H2O TPCTs are about 2-3 times more than that of Cu/H2O TPCTs, moreover the Nusselt's theoretical correlation based on laminar filmwise condensation is not suitable for simulating the h(c) of Ti/H2O TPCTs. Experimental results and theoretical analysis of surface free energy difference between condensate and solid surface indicate that the mixed condensation mode with dropwise and filmwise condensation coexisting on titanium surface result in the higher h(c) for Ti/H2O TPCTs. Experiments on condensation mechanism of titanium surface are ongoing to further validate the point. (C) 2009 Elsevier Ltd. All rights reserved.