Enthalpies of proton transfer in methanol-water mixtures (0-90 wt% methanol) of 3-(N-morpholino)propanesulphonic acid (Mops), N,N-bis(2-hydroxyethyl)-2-aminoethanesulphonic acid (Bes) and N-(2-hydroxyethyl)piperazine-N'-2-ethanesulphonic acid (Hepes) have been measured by direct calorimetry at 25 degrees C. The results thereof are compared with the standard enthalpies for the protonation of the base tris(hydroxymethyl)aminomethane (Tris) and are discussed in terms of an adaptation of classical electrostatic formula and the three-dimensional structure in methanol-water mixtures as determined by molecular dynamic simulations.