The partial molar isentropic pressure coefficients at infinite dilution, K o , have been determined for a number of dipeptides in aqueous solution at 25°C. For a series of dipeptides of sequence gly-X, where X is an amino acid with a neutral side chain, the K o values are all more negative than that for diglycine. The results are discussed in terms of the hydration of the side chains. There are significant differences in the K o values for sequence isomeric dipeptides. These differences can be rationalized in terms of the mutual interactions between the side chain and the ionic end groups in the dipeptides. Possible relationships between K o and V 2 o , the partial molar volume at infinite dilution, were investigated. For the dipeptides of sequence gly-X there is an interesting linear relationship between K o /V 2 o and V 2 o .