Solubilities at 25 degrees C of four amino acids (L-cystine, L-tyrosine, L-leucine, and glycine) in aqueous solutions at various pi-Is and NaCl concentrations (0, 1, 3 mol dm(-3)) are reported. The pH was varied from 0 to 13 using HCl or NaOH. The NaCl concentration strongly affects the solubilities of L-cystine, L-tyrosine, and L-leucine. In particular, the addition of NaCl increases the L-cystine solubility but reduces the solubility of L-tyrosine and L-leucine. The NaCl concentration has only a slight effect on the solubility of glycine. Experimental solubilities are correlated with the use of a chemical equilibrium model, good agreement in the pH range 1-12 was found. The analysis confirms the dependence reported in the literature between the solubility of amino acids and the salt concentration. For the amino acids examined, the parameters to calculate their solubilities in solutions with NaCl are also given.