2.5 and 1.25 per cent gelatin have been titrated potentiometrically in the absence of salts and in the presence of two concentrations (0.0750 and 0.0375µ) of NaCl, MgCl2, K2SO4, and MgSO4. The data have been used to calculate values of ± S = vz – (v – 1)z, where vz = v2 – (v2 – v) rx/18.