Water activity and pH of amino acids (alanine, glicine, arginine and proline) in aqueous and buffer solutions were measured at various concentrations and 25 degreesC. The partial dissociation phenomenon was considered and the UNIFAC model, combined with solvation equations, was used for the water activity and pH predictions. The validity of the model was tested for the aminoacids in water and two buffer solutions. Mean deviations of 0.64% for the water activity and 2.5% for the pH were obtained.