The enthalpies of the mixture of (S)-2-amino-5-guanidinopentanoic acid and (2S)-pyrrolidine-2-carboxylic acid with 1,3-butanediol, 2,3-butanediol, and their respective enthalpies of dilution in aqueous solutions at 310.15 K were determined as a function of the mole fraction by flow microcalorimetric measurements. These experimental results have been analyzed to obtain the heterotactic enthalpic interaction coefficients (h(xy)) according to the McMillan-Mayer theory. It has been found that the h(xy) coefficients between (S)-2amino-5-guanidinopentanoic acid and (2S)-pyrrolidine-2-carboxylic acid with butanediol molecules in aqueous solutions at 310.15 K are all positive. The results are discussed in terms of solute solute-and solute-solvent interactions.