In this work, the electrolyte, amino acid and water solutions are modeled using activity coefficient (AC) approach. Two different models, modified Pitzer ion interaction equation proposed by Esteso (PE) and the Pitzer-Simonson-Clegg model (PSC) are used in their original and modified forms to correlate the activity coefficient and osmotic coefficient data for binary and ternary systems containing water, amino acid/peptide and electrolytes. The considered aqueous systems contain one of the eight amino acids: alanine, glycine, serine, threonine, beta-alanine, methionine, a-amino n-butyric acid and a-amino butyric acid. Also, glycylglycine is used as one peptide. The salts are NaCI, NaBr, NaNO3, KCI and KBr. The results obtained from the studied models are compared to those of experimental data. The comparison shows that the PE model can correlate the experimental AC data of ternary systems more accurate than the other PSC. The equations are tested against the large number (30) of available experimental data sets. (C) 2014 Elsevier B.V. All rights reserved.