The partial molar isobaric expansions at infinite dilution and T = 298.15 K, E-2 degrees {E-2 degrees = (partial derivative V-2 degrees/partial derivative T)(p)} were determined recently for some tripeptides of sequence glycyl-X-glycine, where X is one of the amino acids alanine, valine, leucine, isoleucine, phenylalanine, methionine, proline, serine, threonine. asparagine, glutamine. histidine, and tyrosine. These quantities, along with partial molar heat capacity data available from the literature, were used to convert the partial molar isentropic compressions at infinite dilution determined previously for these tripeptides into the partial molar isothermal compressions at infinite dilution and T = 298.15 K, K-T,K-2 degrees {K-T,K-2 degrees = -(partial derivative V-2 degrees/partial derivative P)(T)}. These K-T,K-2 degrees results, along with that for triglycine, were used to estimate the contributions of the amino acid side-chains to the partial molar isothermal compressions of peptides. The side-chain contributions have been rationalized in terms of the likely peptide-water interactions. (C) 2009 Elsevier Ltd. All rights reserved.