Determination of hydration parameters for the solute-solvent interactions of model peptide molecules can provide quantitative information on the factors affecting the folding and stability of proteins in aqueous solutions. Standard hydration enthalpies are calculated by combination of the standard sublimation and solution enthalpy data, experimentally determined. The results for some N-acetyl amino acid amides, assumed as model for peptides, are reported and the trend of hydration enthalpies with increasing complexity of the model molecules is discussed on the basis of the group additivity method. Further the direct proportionality between hydration enthalpy and non-polar accessible surface area (ASA) of each amino acid residue is emphasized. Finally it is pointed out that there exists a convergence temperature for the enthalpy associated with the hydration process of these model compounds and its value TH* = 93 +/- 7 degrees C is close to that found for small globular proteins (i.e. TH* = 100 +/- 6 degrees C). This finding can give some insights to clarify the emergence of convergence behaviour in the unfolding process.