Abstract The polarographic reduction of cresol red, thymol blue, bromo-cresol purple, bromophenol blue and bromocresol green are studied in buffered aqueous solutions and alcohol—water mixtures of pH 2–11.5. It was found that in solutions of pH values ⩾ 10 the five depolarizers are reduced along two one-electron waves of equal heights. In solutions of pH values <7, bromo-cresol purple and bromophenol blue are reduced through a single two-electron wave. Whereas cresol red, thymol blue and bromo-cresol green are reduced in two one-electron steps. Medium and substituent effects on i 1 and E 1 2 are discussed.