The hysteresis effect observed in the palladium-hydrogen system when the pressure is varied has been investigated by the x-ray method at different temperatures ranging from 60 c. to 130 c. The effect of charging the specimens with hydrogen for long periods was also examined. Definite relations were observed between the temperature and certain critical values of the pressure connected with the appearance and disappearance of the two phases which are present in the system. The α-phase lines in the structure spectra were initially sharply defined, but the β-phase lines were always diffuse, and so also were the α-phase lines when the phase reappeared again on decreasing the pressure of the gas.