Activity-coefficient data were measured for NaCl in the system water-NaCl-DL-valine at 25 degrees C. The activity coefficients of DL-valine were then calculated through rigorous thermodynamic relations. The activity coefficients of the amino acids in this and other aqueous systems containing electrolytes were modeled using an expression for the excess Gibbs free energy of the solution. The model combines a contribution for long-range interactions, given either by the Bromley equation or the K-V equation, and a contribution of short-range interactions represented by a ternary NRTL or Wilson expression. The model accurately correlates the activity coefficients of ten amino acids in aqueous solutions and of six amino acids in aqueous electrolyte solutions.