The Indigo Method is well known for the measurement of ozone in air and exhaust gases. It has now been adapted for the measurement of ozone and chlorine dioxide in drinking water and in other aqueous solutions. This method is faster and of higher precision than the colorimetric methods which are generally applied in waterworks. In the pH region ≤ 4 one mol ozone discolors 1.0 mol of aqueous sulfonated indigo. The stoechiometric factor stays constant even if concentrations varied in the range from 5 μg/l to 10 mg/l. The absorbance at 600 nm changes by ~ 20 000 when based on an addition of 1 mol/l ozone and a 1 cm cell. Chlorine dioxide is determined at pH > 4. The absorbance changes by ~ 7000-9000 per 1 mol/l of added chlorine dioxide. All calibration curves are linear. Reagents and oxidized reagents are quite stable. Hydrogen peroxide, organic peroxides, chlorite, chlorate, manganous ions and oxidized species in drinking water do not interfere with the determination. Chlorine is masked by malonic acid or glycine when ozone respectively chlorine dioxide are to be determined. Good photometers give a precision of 1.5 % or 2 μg/l. Visual methods have a limit of detection of about 10 μg/l.