The coloration reagent - leucomethylene blue, the reduction product of methylene blue, is used to determine the concentration of chlorine dioxide in the presence of Cl2and anion species such as OCl−, ClO2−and ClO3−. This simple spectrophotometric method is performed by using the extractant 1,2-dichloroethane at pH 1.3. The linear range of ClO2measurement extends to 0.95 mgl−1with a detection limit of 0.02 mgl−1. The presence of chlorine and hypochlorite ion can be masked by oxalic acid. The permissible maximum concentration of ClO2−is 2.0 mgl−1, and ClO3−anion does not interfere with the measurement.