There are marked differences in H-2 solubilities between ordered and disordered Pd-Mn alloys with the largest difference found between the L1(2) and the disordered form of the Pd3Mn alloy. The thermodynamics of H2 solution have been determined for the L12 form, the long-period superstructure (1ps), and the disordered forms of the Pd0.80Mn0.20 and Pd0.75Mn0.25(Pd3Mn) alloys. Relative partial molar enthalpies and entropies were determined mainly by reaction calorimetry over the range of H contents accessible from PH2 approximate to 10 Pa to approximate to 0.3 MPa (303 K). The enthalpies for absorption of H2 are more exothermic over most of the range of H contents for the L1(2) forms of the Pd3Mn and Pd0.80Mn0.20 alloys than for their other forms. The reaction enthalpies are constant across a relatively wide range of H contents for the L1(2) form of the Pd0.80Mn0.20 and Pd3Mn alloys indicating that there are two-phase coexistence regions (303 K). The H-H attractive interaction, which leads to hydride formation, is much greater for the L12 than for the other forms of the Pd3Mn alloy and for Pd itself. It has been found that the H-H interaction always decreases in magnitude and, accompanying this, the THS (terminal hydrogen solubility) always increases by alloying Pd.(1) The L1(2) ordered Pd3Mn alloy is an exception to this, and therefore, the generalization about THS must be restricted to disordered face centered cubic (fcc) Pd alloys.