Vapor pressures of aqueous solutions of L-amino acids (glycine, L-alanine, L-valine, and L-serine) were measured at various concentrations at 298 K by a differential pressure static method. Activities and activity coefficients of water were determined. The activity coefficients of the amino acids in water were obtained from the activity coefficients of water using the UNIQUAC equation. The obtained activity coefficients of the amino acids were evaluated by comparison of the values obtained by a conventional virial expansion method and the literature data.