A simple perturbed hard-sphere model has been developed to correlate the activity coefficients of amino acids and peptides in aqueous solutions. The perturbation terms are those due to dispersion forces and dipole-dipole interactions. These interactions are represented by a Lennard-Jones (6-12) and a Keesom expression, respectively. Dipole moments of amino acids and peptides, calculated by a quantum mechanical approach using the Hyperchem molecular modeling software, are reported. The model can accurately correlate the activity coefficients of amino acids and peptides in aqueous solutions. The model was used to correlate and predict the solubilities of amino acids in aqueous solutions at different temperatures. New values of Delta g and Delta h of solution of amino acids are reported.