The presence of pesticides in water sources and their removal by treatment processes is of particular interest currently to water companies and research scientists. Although operators and scientists are debating whether the related standards and legislation are too stringent, the current European Drinking Water Directive stipulates a maximum contaminant level (MCL) of 0.1 μg/L for an individual pesticide. Atrazine is amongst the most frequently identified pesticides in water supply sources. Since conventional treatment processes (chlorination, coagulation and filtration) are unable to reduce this micropollutant to an acceptable concentration, two advanced technologies are being investigated extensively; namely, adsorption onto activated carbon, and ozonation, particularly ozone combined with hydrogen peroxide. As for ozonation, several authors (Glaze et al., 1987; McGuire and Gaston, 1988; Terashima, 1988; Ferguson et al., 1991) have reported that the removal of refractory organics (e.g., 2-methyl isoborneol – MIB and geosmin) by ozone appears to be more effective in natural waters than in pure water solution; this was attributed to the action of natural organic material in water (such as humic substances) which promotes the radical reactions of ozone. In other more fundamental studies (Staehelin and Hoigné, 1985; Xiong and Legube, 1991), humic substances were speculated to be involved in radical decomposition of ozone in solution.