The combination of UVA radiation and ozone, in the presence of titanium dioxide, has been investigated as a potential destructive technology for the treatment of pesticide wastewaters. The contribution of single ozonation (O-3), photolytic ozonation (O-3/UV), and photocatalysis (TiO2/UV) to degradation of carbaryl, a carbamate pesticide is assessed and compared with that of combined photocatalytic ozonation process (TiO2/UV/O-3). For 40 mg/L of carbaryl (500 mL) at pH 6 and with 0.28 g/h of ozone and 1 g/L of TiO2, 92% COD reduction was achieved in 180 min. Under this experimental condition, 76.5% TOC removal and increase of BOD5/COD ratio to 0.38 was also observed. The synergistic effect occurs in the combined photocatalytic ozonation process which is proved by the significant improvement in the mineralization rate constant (0.09 min(-1)) when compared with the sum of mineralization rate constant of photocatalysis and ozonation (0.05 min(-1)). Further synergistic effect was noticed at acidic and neutral PH, but synergism was lost at alkaline PH due to faster self decomposition of ozone.