For many metal/H2 systems values of d (ln p1/2)/d (l/T) are approximately constant over wide temperature ranges for regions of hydrogencontents where two nonstoichiometric condensed phases coexist with the gas. This near constancy is surprising in view of the change in the hydrogen compositions of thetwo con-densed phases with temperature and the dependence of the relative partial molar enthalpies of solution of hydrogen upon the contents of the pure condensedphases. Previous treatments of such systems have not lead to an understanding of the origin of this constancy. In the present research, two approaches are devel-oped for the understanding of the temperature invariance of d (ln p1/2)/d (l/T). One is based on the enthalpy of solution of hydrogen in each of thepure phases adjacent to the phase boundaries and the change of the phase boundaries with temperature. The otherapproach estimates the heats of desorption at various temperatures for the reaction metal hydride-* y2H2+ metal (H saturated) from the relative partial molar enthalpies of desorption. Both approaches are employed in detail for the Pd/H2 system and illustrate the origin of the temperature invariance of d (ln p1/2)/d (l/T).