A series of water-soluble 2-(2'-arylsulfonamidophenyl) benzimidazole derivatives containing electron-donating and accepting groups attached to various positions of the fluorophore π- system has been synthesized and characterized in aqueous solution at 0.1 M ionic strength. The measured p K a's for deprotonation of the sulfonamide group of monosubstituted derivatives range between 6.75 and 9.33 and follow closely Hammett's free energy ...