The evolution of the two-phase thermal control technique is moving in the direction of the use of devices which operate in stable periodic or chaotic unsteady regimes. In these devices the heat transfer coefficient relative to the evaporator walls therefore changes over time and it is hardly predictable, especially in the case of boiling regime. This paper deals with the analysis of the boiling coefficient evolutions over time during the operations of a thermal control devices which periodically operates, the Periodic Two-Phase Thermosyphon (PTPT). An experimental technique for measuring the transient boiling heat transfer coefficient in a thick flat evaporator is shown. This technique has been used to measure boiling heat transfer coefficients over time in the case of evaporators with small pools of liquid (lower than 64 ml) which periodically dries out. The FC72 fluid has been used in the experiments. The results show that no nucleate boiling regime has been observed in the evaporator, which works in an unstable transitional boiling regime with relative heat transfer coefficients lower than those typical of nucleate boiling regimes (lower than 50%). (C) 2008 Elsevier Ltd. All rights reserved.