The kinetics of the decomposition of aqueous ozone in the presence of organic solutes can be described on the basis of a mechanism involving a chain reaction in which ·O2-/HO2· and OH·-radicals act as chain carriers. Such chain reactions are of importance whenever organic solutes are present which produce ·O2-/HO2· species after reacting with OH·-radicals and oxygen. As an example, formic acid/formate significantly accelerate the decomposition of ozone in this way. This is in agreement with the experience of radiation chemists who use formic acid to convert OH·-radicals into ·O2--species. Humic acids can also act as such promoters. Using this model, it is also possible to describe the kinetics of the decomposition of ozone in natural waters where the chain reaction is initiated by hydroxide ions or organic solutes, promoted by organic solutes (humics), and terminated by organic solutes or bicarbonate which act as radical scavengers. The consequences of the application of this model for the prediction of the behaviour of ozone in water treatment processes are discussed.