N-Nitrosodimethylamine (NDMA) is a highly carcinogenic compound that is suspected of carcinogenic activity in the human body. A variety of methods are used to remove NDMA from water, but the main degradation products, dimethylamine (DMA) and NO(2)(-), are also precursors for NDMA formation. UV irradiation combined with ozonation (UV/O(3)) was examined in this investigation for its ability to inhibit the regeneration of NDMA after degradation. Both the degradation products and the regeneration potential of NDMA were compared between UV irradiation alone and UV/O(3). The yields of DMA and NO(2)(-) in the UV/O(3) process were less than for UV irradiation alone. Yields of DMA and NO(2)(-) were 2.25 mg L(-1) and 3.22 mg L(-1) from UV irradiation, while they were 0.92 mg L(-1) and 0.45 mg L(-1) from the UV/O(3) process. Furthermore, the regeneration of NDMA was also less after the UV/O(3) process than after UV irradiation. The concentration of regenerated NDMA was more than 51.8 microg L(-1) after UV irradiation regardless of the dosage of Cl(2). However, the concentration of regenerated NDMA in the UV/O(3) process was less than 7.37 microg L(-1) under the same conditions. Consequently, the UV/O(3) process was more effective than UV irradiation alone in inhibiting NDMA regeneration. The inhibition of NDMA regeneration was due to a decrease in DMA and NO(2)(-) produced by the UV/O(3) process. As the major products generated from NDMA, NO(2)(-) and DMA were likely to be oxidized by ozone and hydroxyl radicals (OH). In addition, the reaction between NDMA and OH would possibly generate methylamine as the only product, leading to a decrease in the production of DMA by the UV/O(3) process.