The relative chemical potential of the dissolved hydrogen (Delta mu(H)(O)) and the partial pair interaction free energies, g(P)(H)<---->(Z), and (g(H)(P)<---->(Z/Z＇))(average) respectively, in Pd based Pd(1-x)Z(x)-H and Pd(1-x-y)Z(x)Z(y)＇-H systems are correlated to the stability of the dissolved hydrogen in the octahedral interstitial sites by classifying the Pd(1-x)Z(x) alloys based on the "effective valence, n(s)" of the substitutional metals. From the dependence of Delta mu(H)(O) on the lattice constant of the alloy, (a(0))(alloys) and of g(H)(P)<---->((g(H)(P)<---->(Z/Z＇))(average)) on the ionic diameter of the substituent metal, d(Z), ((d(Z/Z＇))(average)) for systems Pd(1-x)Z(x)-H (Pd(1-x-y)Z(x)Z(y)＇-H), the exceptional behavior of the binary "expanded" alloys Pd(1-x)Z(x) (Z = Nb, Ta and Pt), the binary "contracted" Pd(1-x)Z(x) alloys (Z = Ti and Li) and the ternary "contracted" alloys Pd0.903Er0.078Cu0.019 and Pd1-2xAgxNix towards hydrogen absorption is explained.