In the previous paper, we described that acetic acid and caproic acid contained in water were decomposed very slowly by treatment of ozone alone, but they were decomposed very rapidly by simultaneous treatment of ozone and UV irradiation. In this report 5 mM aliphatic mono- and di-carboxylic acids in the 0.05 M phosphate buffer (pH 7.0) were treated by ozone + UV, and their degradation pathways were investigated by analysing their decomposition products. Dicarboxylic acids were considered to be produced from each monocarboxylic acid by oxidation at the terminal methyl group. Monocarboxylic acids with small carbon numbers were also produced by oxidative decarboxylation. The decomposition curves of each carboxylic acid indicated straight lines against decomposition time. Therefore, the decomposition reaction was considered to the apparent zero order. The decomposition rate constants of monocarboxylic acids increased with the carbon numbers, and they were decomposed more rapidly, but the rate constants of dicarboxylic acids were nearly equal. Compared to the treatment of ozone alone, the TOC (total organic carbon) removal (%) of malonic acid or adipic acid was very high by treatment of ozone + UV irradiation and very efficient decomposition to inorganic carbon dioxide was assumed. Mutagenicity of O3 + UV treated solutions of caproic acid was assayed with Ames test, and was found to have a little mutagenicity at halfway decomposition time, but it disappeared finally.