AbstractThe mixing behavior of the binary systems Pd ‐ H and Pd ‐ B was previously interpreted in terms of three effects:1.The solute atoms H and B, respectively, occupy octahedral sites.2.They donate their valence electrons to the 4d and 5s bands of the palladium, raising its Fermi energy.3.There exists an energy of attraction between the interstitial particles.This model is now applied to the ternary system Pd ‐ B ‐ H. The Fermi energy contribution to the thermodynamic properties is treated strictly as a known function of the total valence electron concentration, which is in agreement with magnetic susceptibility measurements on these systems.The quasichemical approach, which was used to calculate the thermodynamic properties connected with the energy of attraction in the binary systems, is extended to the three‐component system Pd ‐ B ‐ H. In addition to the known energies of attraction between two like particles there appears a third parameter, the energy of attraction between unlike particles.The method is applied to hydrogen absorption measurements on Pd ‐ B alloys. Good fit is obtained with one adjustable parameter, the interaction energy between unlike particles. An interpretation is given for the three energies of attraction in terms of a lowering of the lattice strain energy for nearest neighbor int