The dissociation constants of 3-benzamidorhodanine and its substituted derivatives have been determined potentiometrically and spectrophotometrically. The data are discussed in terms of the electronic character of the substituents and of the change in temperature and medium of the solution. The pK(H) values have been found to increase with increasing electron-repelling nature of the substituents and with decreasing ionic strength and dielectric constant of the medium. The resulting linear Hammett plots of pK(H) versus the Hammett constant sigma values indicates the coplanarity of the molecules of the investigated compounds. The dissociation constants have been found to depend on the nature of the solvent as well as on its content. The evaluated thermodynamic parameters indicate that the dissociation processes are non-spontaneous, endothermic, and entropically unfavourable. These functions are affected by the electronic character of the substituents.