Differences in the degradation and transformation of atrazine (ATZ), carbamazepine (CBZ), diclofenac (DCF), and sulfamethoxazole (SMX) in deionized water during UV and UV/H2O2 treatment using low-pressure (LP) and medium pressure (MP) UV lamps, were assessed using a collimated beam apparatus. UV doses ranged from 300-700 mJ/cm(2) and H2O2 doses ranged from 0-10 mg/L. Liquid chromatography with tandem mass spectrometry (LC-MS/MS) was used to measure concentrations of the parent compounds and quadrupole time-of-flight mass spectrometry (QToF-MS) was used to screen for transformation products following treatment. In general, there was little difference in compound degradation and transformation between LP and MP UV lamps in both UV and UV/H2O2 treatments. Removal of ATZ, SMX and DCF was largely attributed to direct photodegradation whereas CBZ was not appreciably removed by UV or UV/H2O2 treatment. All four compounds yielded transformation products following UV or UV/H2O2 treatment with LP and MP lamps. Transformation pathways were determined using accurate mass estimation to determine elemental composition, and relative abundance was determined using ion counts. For ATZ and CBZ, the transformation pathway was non-sequential, whereas for DCF and SMX, the transformation pathway was sequential. The approach outlined in this paper can be used to assess unknown transformation products formed during oxidation of organic micropollutants during water treatment. (C) 2012 Elsevier B.V. All rights reserved.