Dielectric barrier discharges (DBD) were used for the degradation of carbamazepine (CBZ) in aqueous solution. The electric discharge was generated either ex situ or in situ directly on the water surface. To maintain the same ozone concentration of 40 ppm in both instances, the power injected was 0.7W in the ex situ discharge and 12W in the in situ discharge. The results showed 100% CBZ removal after 3 min of treatment with the ex situ discharge, while the in situ discharge only removed 81% of the CBZ after 60 min. According to measurements of UV absorbance at 285 nm and 254 nm. and of total organic carbon, the ex situ discharge system also proved to be more effective than the in situ system. The measurement of nitrogen oxides in both gaseous and liquid phases indicated that high energy in situ discharges produced a large amount of NOx. These species contributed to decreased pH and significantly slowed the CBZ oxidation rate, due to their competition with ozone. Production of NOx should be avoided when using the DBD technique for wastewater treatment. (C) 2012 Elsevier B.V. All rights reserved.