The density of aqueous solutions of amino acids has been modeled with the statistical associating fluid theory (SAFT) equation of state. The modeling is accomplished by extending the previously developed new method to determine the SAFT parameters for amino acids. The modeled systems include alpha-alanine/H2O, beta-alanine/H2O, proline/H2O, L-asparagine/H2O, L-glutamine/H2O, L-histidine/H2O, serine/H2O, glycine/H2O, alanine/H2O/sucrose, DL-valine/H2O/sucrose, arginine/H2O/Sucrose, serine/H2O/ethylene glycol, and glycine/ H2O/ethylene glycol. The density of binary solutions of amino acids has been correlated or predicted with a high precision. And then the density of multicomponent aqueous solutions of amino acids has been modeled based on the modeling results of binary systems, and a high accuracy of density calculations has been obtained. Finally, the water activities of DL-valine/H2O, glycine/H2O, and proline/H2O have been predicted without using binary interaction parameters, and good results have been obtained. (C) 2007 Elsevier Ltd. All rights reserved.