Abstract In this paper new results are given concerning the influence of valence fluctuation on the thermodynamic stability of the TmSe system. Initial measurements of the determination, by fluorine combustion calorimetry, of the enthalpy of formation of Tm x Se as a function of the nonstoichiometry are presented. A discussion of the apparatus is given. It is shown that a miscibility gap exists between the normal metallic TmSe phase and the valence fluctuating phase. The repulsive forces are probably due to the very different valence states of the two phases. Two additional intermediate phases are indicated by fluorine combustion calorimetry. Samples quenched from high temperatures reveal the co-existence of two phases: Tm 0.94Se (normal metallic) and Tm 1.05Se (valence fluctuating). These phases react during cooling to give the corresponding solid solutions with lattice constants following Végard's law. These results can be explained by the existence of a wide miscibility gap (0.94 < x < 1.05) at high temperatures which contracts appreciably (0.94 < x < 0.98) at low temperatures. Both the above results, as well as those from DTA, show the existence of severe kinetic hindrances. This can be explained by a low diffusion coefficient of the fluctuating phase.