Enthalpies for the two proton ionizations of glycine, N,N-bis(2-hyroxyethyl)glycine ("bicine") and N-tris(hydroxymethyl)methylglycine ("tricine") 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 glycine increased from 4.4 to 9.4 kJ mol(-1) with a minimum of 4.10 mol(-1) at X-m = 0.059. The ionization enthalpy of the second proton (Delta H-2) for glycine decreased from 46.3 to 38.1 kJ mol(-1). Delta H-1 of bicine increased from 3.5 to 7.6 kJ mol(-1) at X-m = 0.273 before dropping to 4.1 kJ mol(-1) at X-m = 0.360. Delta H-2 of bicine increased from 24.9 to 29.4 kJ mol(-1). For tricine, Delta H-1 increased from 6.7 to 9.8 kJ mol(-1) at X-m = 0.194 then dropped to 7.4 kJ mol(-1) at X-m = 0.360. Delta H-2 for tricine first dropped from 30.8 to 28.5 kJ mol(-1) at X-m = 0.059 before increasing to 33.3 kJ mol(-1) at X-m = 0.273. 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.