The dissociation constant values of l-histidine, proline and tryptophane were determined at 25 ± 0.1°C by potentiometric pH titration in pure water and different hydroorganic solvent media. The organic solvents used were methanol, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, acetone and dioxane. Initial estimates of the dissociation constant values of the different amino acids studied have been refined with ESAP2M computer program. It was observed that changing the medium permittivity as the solvent is enriched in methanol or ethanol has little influence on the pK*a values of the amino acids studied. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents and the possible variation in microheterogeneity of the salvation shells around the solute. © 1994.