Lines of formation and decomposition of the hcp (ε) molybdenum deuteride in the T–P phase diagram of the Mo–D system are constructed via the measurement of isotherms of the electrical resistance of molybdenum in a deuterium atmosphere at temperatures up to 550 °C and pressures up to 6 GPa. The diagram is compared to that of the Mo–H system studied earlier. The pressure of phase equilibrium in both systems is shown to be much closer to the decomposition pressure than to the formation pressure. A new explanation is suggested for this asymmetry in the hysteresis loop that seems to be characteristic of many metal–hydrogen systems. From the position of the decomposition lines, the standard entropy, enthalpy and Gibbs energy of formation are estimated for molybdenum deuteride and hydride.