An alternative to traditional modifications of the Debye-Huckel equation by using added semiempirical terms of the ionic strength-the mean spherical approximation (MSA)-is used to derive equations which allow to calculate the effect of the ionic strength on the stoichiometric protonation constants of glycine in KNO3 and NaCl media. Calculations with the MSA model entail the knowledge of the charge and diameter of the different ionic species, Charges are accurately determined by the model, but diameters are not. As regards the diameters of the ions making the electrolytes used, we chose those previously employed by other authors. On the contrary, there are few reported values far the diameter of the different ionic species resulting from the amino acid ionization so we calculated the van der Waals volume and used it as a rough approximation to the diameter of both the cation (AH(2)(+)) and the anion (A(-)). Such a value was modified so as to study its influence on the contribution of both the electrostatic term of activity coefficients and the hard sphere one. The pK(i)(*) vs electrolyte concentration curves obtained closely fit experimental data within the experimental error range for potentiometric measurements.