The insensitive munitions compound 2,4-dinitroanisole (DNAN) is increasingly being used as a replacement for traditional, sensitive munitions compounds (e.g., trinitrotoluene [TNT]), but the environmental fate and photo-transformation of DNAN in natural water systems are currently unknown. In this study, we investigated the photo-transformation rates of DNAN with both ultraviolet (UV) and sunlight irradiation under different environmentally relevant conditions. Sunlight photo-transformation of DNAN in water was found to follow predominantly pseudo-first-order decay kinetics with an average half-life (t(1/2)) of approximately 0.70d and activation energy (E(a)) of 53kJmol(-1). Photo-transformation rates of DNAN were dependent on the wavelength of the light source: irradiation with UV-B light (280-315nm) resulted in a greater quantum yield of transformation (ϕ(UV-B)=3.7×10(-4)) than rates obtained with UV-A light (ϕ(UV-A)=2.9×10(-4) at 316-400nm) and sunlight (ϕ(sun)=1.1×10(-4)). Photo-oxidation was the dominant mechanism for DNAN photo-transformation, based on the formation of nitrite (NO(2)(-)) and nitrate (NO(3)(-)) as major N species and 2,4-dinitrophenol as the minor species. Environmental factors (e.g., temperature, pH, and the presence or absence of naturally dissolved organic matter) displayed modest to little effects on the rate of DNAN photo-transformation. These observations indicate that sunlight-induced photo-transformation of DNAN may represent a significant abiotic degradation pathway in surface water, which may have important implications in evaluating the potential impacts and risks of DNAN in the environment.