Enthalpies for the two proton ionizations of the biochemical buffers N-[2-hydroxyethyl]piperazine-N'-[2-ethane sulfonic acid] (HEPES) and N-[2-hydroxyethyl]piperazine-N'-[2-hydroxypropane sulfonic acid] (HEPPSO) were obtained in water-methanol mixtures with methanol mole fraction (X-m) from 0 to 0.360. With increasing methanol, the ionization enthalpy for the first proton (Delta H-1) of HEPES increased steadily from 8.4 to 15.3 kJ mol(-1) whereas that for HEPPSO rose to a maximum of 21.00 mol(-1) at X-m =0.123 before dropping to 18.4 kJ mol(-1) at X-m =0.360. The ionization enthalpy for the second proton (Delta H-2) of HEPES varied from 20.8 kJ mol(-1) in water to 13.6 kJ mol(-1) at X-m =0.360 with a maximum of 24.8 kJ mol(-1) at X-m = 0.194. For HEPPSO, Delta H-2 increased steadily from 23.4 to 29.2 kJ mol(-1). The solvent composition was selected so as to include the region of maximum structure enhancement of water by methanol. The results were interpreted in terms of solvent-solvent and solvent-solute interactions. (C) 2006 Elsevier B.V. All rights reserved.