The thermodynamic dissociation constants for the isoelectric reaction LH+ ⇌ L + H+ (where L = 1,10-phenanthroline) were determined pH-metrically and spectrophotometrically in ethanediol + water mixtures (0-100%). The changes in the pK values are interpreted in terms of solvent basicity and ion-solvent interactions. The free energies of transfer of the H+ ion from water to ethanediol + water mixtures were also calculated. The results suggest that the ethanediol + water mixtures are more basic than water. The basicity increases and passes through a maximum at about 60 wt.% of ethanediol. However, the ethanediol and ethanediol-rich mixtures are less basic than water. The changes in basicity are probably related to changes in the structure of the solvent mixtures. © 1989.