Acid-base equilibria were studied potentiometrically in (phenol + phenolate) systems involving nine substituted phenols with both electron-donating and electron-accepting substituents in a polar protophobic aprotic solvent, acetonitrile. To estimate the influence of the polarity of the medium on acid dissociation and anionic homoconjugation equilibria, the equilibrium constants were also determined in an (acetonitrile+cyclohexane) solvent in which the latter was a diluent that lowered the dielectric permittivity of the reaction medium, and compared with those determined in acetonitrile.