Photolysis of isoxaben N-[3-(1-ethyl-l-methylpropyl)-isoxazol-5-yl]-2-6-dimethoxylbenzamide in dilute aqueous solutions, adsorbed on silica gel plates and in solid phase was studied. The rate of photodecomposition was determined. The results suggest that the rate is not first order. Some photoproducts were isolated and identified by spectroscopic methods. Photo-decomposition with sunlight leads to the formation of three isomers, 2,6-dimethoxybenzo-nitrile and 2,6-dimethoxybenzamide as major photoproducts. The rearrangement has been shown to proceed in two photochemical steps by way of azirine II and unidentified photoproduct IV as stable intermediates. Azirine II is photodegraded to the product oxazole III and rearranges back to the starting isoxaben I. The same degradation products were found when isoxaben was adsorbed on silica gel and in solid phase.