The partial molar isentropic pressure coefficients at infinite dilution K s,2 ∞ ( K s,2 ∞ = −(∂ V s,2 t8/∂ p) s, where V 2 ∞ is the partial molar volume at infinite dilution) have been determined for nine tripeptides in aqueous solution at 25°C. The tripeptides are all of sequence glycyl- X-glycine, where X is an amino acid. These K s,2 ∞ results, in conjunction with those for glycylglycylglycine, were used to estimate the amino acid side-chain contributions to K s,2 ∞ of peptides. These side-chain contributions are critically compared with previous estimates based on K s,2 ∞ data for the amino acids. The variation in the side-chain contributions derived using the peptide K s,2 ∞ results has been rationalized in terms of likely peptide-solvent interactions.