Abstract: The modified Wilson model have been used to correlate the activity coefficients of amino acids and simple peptides in aqueous solutions. In the models studied in this reserch only the binary interaction parameters for amino acid-water molecules were taken into account. This treatment was based on this phenomenon that amino acid molecules are in their zweitter ionic in aqueous solutions. The results obtained from the model were compared with the experimental data of the activity coefficients in the literature. It is shown that this model can accurately correlate the experimental data for activity coefficients. The models were also used to correlate the solubility of amino acids in aqueous solutions. The results show that this model can accurately correlate the solubility experimental data over a wide range of temperature. The values for Δh, Δs and Δg of the solutions were calculated from the results of this model.