An electrochemical method was used to measure the mean activity coefficients of amino acids in water-electrolyte-amino acid systems. The amino acids were glycine, DL-serine and DL-valine and the electrolytes were KNO3 and NaNO3. The activity coefficients of the electrolytes were obtained from the e.m.f measurements in an electrochemical cell with an anion and a cation ion-selective electrodes, each measured versus a double junction reference electrode at 298.15 K. The activity coefficients of the amino acids in the ternary systems were obtained from those of the electrolyte according to the cross differential relation. A simple semi-empirical model based on a modification of a more complex form, derived from perturbation theory, was used to correlate the activity coefficients in binary aqueous solutions and in ternary systems.