Under conditions of drinking water treatment, chlorine dioxide reacts with individual solutes according to rate laws which are 1st order with respect to the concentration of both chlorine dioxide and the solute. Its reactivity can therefore be best characterized by 2nd-order reaction-rate constants. These constants are high in the case of phenolic compounds (which only react according to their degree of dissociation to phenolate ions), non-protonated tertiary amines, thio-compounds,and somewhat lower for inorganic compounds such as nitrite, iodide or ozone. Unreactive are olefinic C = C double bonds, aromatic hydrocarbons, ammonia and amines (except tertiary), amino acids (except those containing a sulphur group), aldehydes, ketones, quinone, carbohydrates, and bromide ion. The latter compounds cannot react significantly under water treatment conditions. The difference in selectivity between ozone and chlorine dioxide is illustrated by comparing their respective rate constants.