For the modeling of activity coefficients and solubility of amino acids that are predominantly present as zwitterions in neutral aqueous solutions, excess Gibbs energy (G(E)) models and Helmholtz free energy models in which solvent is considered as a dielectric continuum have been used. In the present study, charges in zwitterions were considered as proton donors and acceptors. The hydrogen-bonding nonrandom lattice fluid equation of state (EOS) was adopted to account for the specific interactions as well as physical interactions. Parameters were fitted to vapor pressure and density for water and to activity coefficients and partial molar volumes at infinite dilution for amino acids. The calculated densities, activity coefficients and solubilities were in good agreements with data. (C) 2003 Elsevier B.V. All rights reserved.