In this paper, the activity coefficients of amino acids and simple peptides in aqueous solutions were correlated by using a three parameters model based on the perturbation theory. The adjustable parameters of this model were obtained from the experimental data and a relation for calculating the activity coefficient is derived. The calculated activity coefficients of amino acids and simple peptides obtained show that the equation of state based on the perturbation model can be used to correlate the activity coefficients of-amino acids more accurately than the other models. A correlation for the solubility of amino acids in aqueous solutions is also derived. The results show that this correlation can accurately correlate the solubility of amino acids in aqueous solution over a wide range of temperatures (0-100 degrees C). (c) 2005 Elsevier B.V. All rights reserved.