The osmotic coefficients of glycine, dl-α-alanine and dl-α-aminobutyric acid in water were measured at 288.15, 293.15, 298.15 and 303.15K using the isopiestic method. The activity coefficients were calculated for the amino acids and the pairwise free energy for solute–solute interactions were adjusted according to the McMillan–Mayer theory. The osmotic and activity coefficients of amino acids are compared with literature data when they are available and the results are discussed in terms of solute–solute interactions.