Pendimethalin (I) decomposed in hexane, methanol and aqueous methanol upon irradiation with UV-light (λ > 290 nm) to form "polar" products. The main products formed are the dinitroaniline (II) and the isomers mono-nitroanilines (III). Kinetic studies indicate that the photodegradation of pendimethalin is strongly dependent of solvent and concentration.