An all-metal facility has been built to handle about 1000 Ci of tritium for studies of the tritium behavior in metals. First measurements of the pressure-composition-temperature relationship for the palladium-tritium system Pd T x are reported for the pressure range 0.1≤ p≤ 2 bar, the composition range 0.02≤ x≤ 0.65, and the temperature range 35≤ T≤ 125 C together with the results of the PdH and PdD systems measured under the same experimental conditions. The main conclusions are that the inequality p T 2> p D 2> p H 2 holds at equal concentrations and temperatures and that the β→ α transition enthalpies show a strong isotopic effect, whereas the entropies do not.