The effect of radical scavengers such as bicarbonate and tert-butanol on the Mn^II catalysed ozonation of atrazine, an important herbicide and well-established radical probe substance, was studied using a conventional gas bubble-contacting column. It was found that the presence of a small amount of Mn^II (0.3–1.2 mg/l) greatly increased the degradation rate of atrazine, with the formation of by-product compounds of a lower molecular weight and a greater polarity (as indicated by shorter retention times in HPLC chromatography). However, the presence of either bicarbonate or tert-butanol had a negative effect on the degradation of atrazine. With an increase of bicarbonate concentration, the oxidation rate of atrazine by Mn^II-catalysed ozone was substantially reduced and a correspondingly higher residual ozone was observed. In the presence of tert-butanol, greater reductions of the degradation rate of atrazine were observed and the decomposition of ozone was greatly retarded, resulting in a higher residual ozone; this was the case for either ozone oxidation alone or Mn^II-catalysed ozone oxidation. These results appear to confirm that the degradation of atrazine by ozone in the presence of Mn^II follows a radical mechanism. It is believed that Mn^II catalyses the decomposition of ozone through the formation of intermediate manganese species (such as Mn^IV), leading to the generation of hydroxyl radicals.