Nitrosodimethylamine (NDMA), a potent carcinogen, is found in natural and engineered water systems. Ultraviolet (UV) treatment is potentially a useful tool for controlling NDMA at water purification plants. However, a recent study has reported that UV/advanced oxidation process (AOP) treatment may provide a source of NDMA precursors. Therefore, we investigated the effect of UV treatment on the potential for NDMA formation. Synthetic water samples containing NDMA together with Suwannee River Natural Organic Matter (SRNOM) and/or nitrate were exposed to a low-pressure (LP) UV lamp, a medium-pressure (MP) UV lamp, or a KrCl excimer lamp to determine the spectral impacts. NDMA formation potential was significantly decreased in nitrate solution by LP or MP UV exposure. Nitrate exposed to UV was assumed to have produced the OH radical, which degraded NDMA precursors. The NDMA concentration increased in solutions exposed to UV, even for those without chloramination, which contributed a certain portion of the total NDMA formation. Exposure to MP UV affected the formation of NDMA more significantly than exposure to LP UV due to the overlap of wavelengths in the MP UV emission and the absorption of nitrate.