The pK(a) values of the monocation-neutral equilibrium of five benzimidazoles have been studied in anionic micelle (sodium dodecyl sulfate, SDS), nonionic micelle (Tween-80), and dioxane-water mixtures of different compositions. The disagreement between the pK(a) value observed in water and in Tween-80 is due to smaller dielectric constant (D-eff) at the interface, whereas in the case of SDS micelles it is due to smaller D-eff, surface potentials, and specific molecular interaction between cationic proton and sulfate head group. Orientation of the heterocyclic nitrogen atom, in the case of 2-phenylbenzimidazole also, plays a role in the above discrepancy.