BACKGROUND: The effectiveness of UV radiation on the indirect photodegradation of metronidazole (MNZ) conducting a kinetic study of the process in the presence of radical promoter (H2O2) and photosensitizer (Sens) promoters (duroquinone, tetrahydroxyquinone, tetrachloro-1,4-benzoquinone, 4-carboxy-benzophenone, 2,4-dihydroxybenzophenone, 4-hydroxybenzophenone, and 4,4'-dihydroxybenzophenone) was investigated. RESULTS: The electron-donating groups of photosensitizers may favor MNZ photodegradation through electron transfer. However, the photosensitizers studied showed low effectiveness of MNZ photodegradation in aqueous phase. The presence of H2O2 substantially increased MNZ photodegradation rate, which was enhanced by increasing the concentration of hydrogen peroxide in the system. The contribution of direct photolysis of MNZ in the UV/H2O2 system was drastically decreased with increasing H2O2 concentrations. CONCLUSIONS: The low effectiveness obtained with most of the Sens used demonstrates that metronidazole cannot be photoxidized by indirect photooxidation in aqueous solution. Copyright (C) 2012 Society of Chemical Industry