Introduction Direct absorption of hydrogen by palladium from hydrogen-stirred acidic water solutions has been shown to occur readily. 1’2 3It has been demonstrated that electrode potentials of thepalladium specimen during the course of absorption from solution have thermodynamic significance, ie, the hydrogen is approximately uniformly distributed throughout each phase of the nonstoichiometric hydrides during absorption. 2· 8 Absorption of hydrogen from solution is a useful technique for the determination of absorption isotherms below 100 because direct equilibration in the absence of solution is slow below 100. Carson, et., have employed this technique to investigate the absorption of hydrogen by a series of palladium-rich platinum-palladium alloys. The equilibrium solubilities (25, 1 atm.) of hydrogen in these alloys were found to de-crease with increasing platinum content. Absorption isotherms revealed the existence of pressure-invariant regions for alloys with at least 8.80% platinum (25, 1 atm.). In analogy with the behavior of pure palla-dium, 6 these pressure invariant regions correspond to the coexistence of the a- and/3-nonstoichiometric hydride phases. The two-phase region was said to disappear when the platinum content had reached 12±1.5%, but the isotherm obtained for the 12.03% platinum-palladium alloy was inconclusive. 4 A large difference in the equilibrium isotopic solu-