Partial molar volumes, V ₂ ^o , and partial molar heat capacities, C _p,2 ^o , of the tripeptides glycylglycylglycine, glycylglycylalanine, glycylalanylglycine and alanylglycylglycine have been determined in aqueous solution at 25°C. For the three alanyl-containing tripeptides, the data indicate that the tripeptide-water interaction is influenced by the side chain position within the molecule. The results have been rationalized in terms of likely solutesolvent interactions. The V ₂ ^o and C _p.2 ^o data have also been used to calculate the contribution to these properties of a-CH₃ side chain.