Enthalpies of dilution for ternary aqueous solutions of glycine, glycyl-glycine, triglycine, and L-alpha-alanyl-L-alpha-alanine with caffeine, as well as of glycyl-glycine, and L-alpha-alanyl-L-alpha-alanine with 1,3-dimethyluracil have been determined. The results have been analysed in terms of the McMillan-Mayer formalism in order to obtain the enthalpic virial coefficients. Weak interactions were observed. The densities and molar volumes at infinite dilution were determined for the systems mentioned, as well as for aqueous solutions of glycine, glycyl-glycine and triglycine.