This study investigates the condensation processes in the vertical tube condenser of a loop thermosyphon. For visualization, a glass condensation tube with an untreated (static contact angle of 44 degrees) or a hydrophobically-treated (static contact angle of 86 degrees) inner surface is tested. To avoid the interference from unstable boiling, the evaporator is designed to operate with stable thin-film evaporation. The condensation process in the untreated glass tube is in a composite dropwise-film condensation mode. It was essentially dropwise condensation in the upper section. In the middle and lower sections, the condensing drops were intermittently swept down by the liquid streams falling from the upper section. The lower section was mostly covered with liquid films with circumferentially non-uniform thickness. The condensation mode in the hydrophobically-treated tube is dropwise condensation, for which the condenser resistances are less than for untreated glass surface by 1-2 degrees C-cm(2)/W. No nucleate boiling is observed in the evaporator undergoing stable, quiescent thin-film evaporation. The water levels in the evaporator are nearly the same for all heat loads. The evaporator resistances range from 0.28-0.14 degrees C-cm(2)/W for heat loads of 35.6-92.5W. (C) 2015 Elsevier Ltd. All rights reserved.