In the ozonation of water, ozone may decompose and form hydroxyl radicals (OH·). These very reactive intermediates are the most important oxidants for many solutes, but they are even scavenged by carbonate- and bicarbonate ions. Thus, many types of organic trace-impurities in waters are protected by the carbonate species from the oxidation by OH·. Studies on solutions of benzene, tetrachloroethylene, methanol and ammonia (0,1-4 mg/l) as model solutes give results in fair agreement with those expected from known reaction-rate constants. Furthermore the effect of CO3- is about 12 times greater than that of HCO3-. The scavenging reaction of carbonate also inhibits a radical chain reaction by which the O3-decomposition is accelerated by many organic trace solutes. Thus, carbonate stabilizes O3 and thereby intensifies the direct reactions of O3-molecules with solutes. Measurements on drinking water from different sources (raw water) demonstrate the stabilizing effect of bicarbonate on O3 and its inhibition on the oxidation of trace impurities. Parallel studies on the scavenging of radiolytically initiated OH· by carbonate were used for comparisons with results obtained with ozone.