Systematic determinations of electrical conductivities of sodium salts of semisynthetic penicillins, oxacillin, cloxacillin, dicloxacillin and nafcillin in the 278.15K to 313.15K temperature range are reported. These conductivities are examined by applying the Quint–Viallard conductivity equations and the Debye–Hückel equations for activity coefficients. Determined dissociation constants and the limiting conductances of involved anions are evaluated basing on the assumption that in dilute aqueous solutions these salts, similarly as sodium salts of natural penicillins, behave as acidic salts of dibasic acids which are the final products of degradation reactions in acidic media.