Spectrophotometric determination of chlorine dioxide in water and wastewater is now possible for the ranges 0-0.1 mg/L, 0-0.2 mg/L, and 0-1.0 mg/L. The method is a modification of a procedure reported in 1978, which determines Cl02 concentration by the oxidation of chlorophenol red, a pH indicator. Includes 11 references, figures.