The partial molar heat capacities of the tripeptides of sequence glycyl-X-glycine, where X is one of the amino acids glycine, alanine, valine, serine, tyrosine, lysine and aspartic acid, in aqueous solution have been determined over the 10-100 degrees C range, using high sensitivity scanning microcalorimetry. Using these results, the partial molar heat capacities of the various amino acid side-chains were derived. The values obtained differ significantly from literature data that were derived using partial molar heat capacities for small organic compounds that model the Various amino acid side-chains. A rationale has been proposed to account for the observed differences between the side-chain heat capacities derived using partial molar heat capacity data for the tripeptides and those derived using the partial molar heat capacities of the organic analogues.