The relationships between equilibrium hydrogen pressure, hydrogen content, and temperature have been derived for nonstoichiometric transition metal hydrides from both statistical mechanical and thermodynamic considerations. By comparing the derived equations with experimental pressure‐composition isotherms, the type of lattice defect causing deviations from stoichiometry as well as the energies of defect formation and interaction can be calculated. This was done for uranium hydride and palladium hydride. The energies obtained were 69 kcal/mole for the vacancy formation energy, and 4.4 kcal/mole for the attractive vacancy interaction energy in uranium hydride. The corresponding energies in palladium hydride were 58.0 kcal/mole and 0.35 kcal/mole. The derived relationships are of general applicability to any nonstoichiometric binary compound which is deficient in the volatile component.