Isotherms of the palladiumfhydrogen system have been determined up to 990 atm at supercritical temperatures from 326" to 477 C. At the highest pressures the atomic ratio H/Pd in the solid phase exceeds 0.59, assumed as a limiting value in Lacher's theory although the theoretical equation reproduces the general features of the isotherms, which show the persistence of H+ H interaction above the critical region. The new isotherms are correlated with existing measurements for the critical region and for the same pressure range at low temperatures. The electrical resistance of the solid phase has also been measured.

Previous investigations of the Pd+ hydrogen equilibrium have provided accurate isotherms up to and slightly above the critical point for the co-existence of two solid phases, which occurs at 295 C and about 20 atm (H/Pd= 0.27). The measurements described here were made to study the equilibrium over a wide range of solid phase composition at supercritical temperatures. No previous study of this region has been reported except for a few experiments by Dewar. 1