The enthalpies of solution in water, A(sol)H(m), of some small peptides, namely the amides of five N-acetyl substituted amino acids of glycine, L-alanine, L-proline, L-valine, L-leucine and two cyclic anhydrides of glycine and L-sarcosine (diketopiperazines), were measured by isothermal calorimetry at T = (296.84, 306.89, and 316.95) K. The enthalpies of solution at infinite dilution at T = 298.15 K were derived and added to the enthalpies of sublimation, Delta H-sub(m)o, at the same temperature, to obtain the corresponding solvation enthalpies at infinite dilution, A(solv)H(m)(infinity). Moreover, the partial molar heat capacities at infinite dilution at T = 298.15 K, C-p,2(infinity), were calculated by add ing molar heat capacities of solid small peptides, C-p,C-n,(cr), to the Delta C-sol(p,m)infinity values obtained from our experimental data. CH2 group contributions, in terms of solvation enthalpy and partial molar heat capacity, were -3.2 kJ center dot mol(-1) and 89.3 J center dot K-1 center dot mol(-1), respectively, in good agreement with the literature data. Simple additive methods were used to estimate the average molar enthalpy of solvation and partial molar heat capacity at infinite dilution for the 1/2CONH...CONH functional group in the small peptides. Values obtained were -46.7 kJ center dot mol(-1) for solvation enthalpy and -42.4 J center dot K-1 center dot mol(-1) for partial molar heat capacity, significantly lower than values obtained for the CONH functional group in monofunctional model compounds. (c) 2005 Elsevier Ltd. All rights reserved.