The solubility of hydrogen in the vacuum-annealed form of Pd₃Mn, i.e., the long-period superstructure form, is much greater than that in the disordered form of the same alloy as determined from solubility measurements below the order-disorder temperature. For example, at 400 K the solubility in the ordered form is approximately=18 times greater than that in the disordered form at the same, small hydrogen pressure. This difference in solubilities is principally due to the greater exothermicity of hydrogen solution in the ordered form. The optic model frequencies of protium and deuterium have been measured by inelastic neutron scattering and the frequencies for both forms of the alloy are found to be somewhat greater than for pure palladium-hydrogen ( alpha -phase) and they are consistent with the occupation of octahedral interstices. The observed second harmonics and isotope shifts show that the local potential for hydrogen bound in both the ordered and disordered forms is highly anharmonic, resembling that of the usual potential found for hydrogen in pure palladium.