The osmotic coefficients of acetamide, propionamide and butyramide in water were measured at 293.15, 298.15 and 308.15 K using the isopiestic method. The activity coefficients were calculated for the aliphatic amides and the pairwise free energy coefficients for solute-solute interactions were determined according to the McMillan-Mayer theory. The osmotic and activity coefficients of amides are discussed in terms of solute-solute interactions.