The water activity, pH and density of some aqueous amino acid solutions were determined at 25 degreesC in three different types of solvents. Previous published experimental data on water activity and solubility of amino acids in aqueous solutions were used together with data from this work to test the applicability of a group contribution model. The activity coefficients were estimated by the UNIFAC-Larsen model combined with the Debye-Huckel equation, taking also into account the partial dissociation phenomena of species in solution. Interaction energies between the charged species Na+ and Cl-and the specific groups of amino acids (COOH and NH2) were adjusted using experimental solubility data.