The stabilities and the heats of formation of silver(I) tributylphosphine, tricyclohexylphosphine, dicyclohexylphosphine, and tributylstibine complexes have been determined in pyridine, by means of potentiometric and calorimetric measurements. Three mononuclear complexes are formed with tributylphosphine, while only two mononuclear complexes are formed with the other ligands, within the ranges of concentration available. All complexes are formed in strongly exothermic reactions, while the entropy changes are negative. The stabilities of the silver(I) phosphine complexes show a strong correlation with some physical parameters of the ligands, such as the dipole moments, and the acid constants in aqueous solution, and the electronic parameters, v, determtned IR-spectroscopically in the Ni(CO) 3L complexes, L=phosphine or phosphite, in dichloromethane. The stabilities of the trialkylphosphine complexes are higher than the triphenylphosphine complexes, and the stabilities of alkylphosphines are in the order (primary) < secondary < tertiary phosphines.