The initial phase of ozone decomposition in natural water (t < 20 s) is poorly understood. It has recently been shown to result in very high transient HO center dot concentrations and, thereby, plays an essential role during processes such as bromate formation or contaminants oxidation. Phenols and amines are ubiquitous moieties of natural organic matter. Naturally occurring concentrations of primary, secondary, and tertiary amines, amino acids, and phenol were added to surface water, and ozone decomposition as well as HO center dot generation were measured starting 350 milliseconds after ozone addition. Six seconds into the process, 5,mu M of dimethylamine and phenol had generated integral HO(center dot)dt = 1 x 10(-10) M.s and 1.8 x 10(-10) M.s, respectively. With 10 mu M dimethylamine and 1.5 mgO(3)/L, R-ct, (integral HO(center dot)dt/integral O(3)dt) reached 10(-6), which is larger than in advanced oxidation processes (AOP) such as O-3/H2O2. Experiments in the presence of HO center dot-scavengers inclicated that a significant fraction of phenol-induced ozone decomposition and HO center dot generation results from a direct electron transfer to ozone. For dimethylamine, the main mechanism of HO, generation is direct formation of O-2(center dot-) which reacts selectively with O-3 to form O-3(center dot-). Pretreatment of phenol-containing water with HOCI or HOBr did not decrease HO center dot generation, while the same treatment of dimethylamine-containing water considerably reduced HO center dot generation.