The photodegradation of the organophosphorus pesticide azinphos methyl and 1,2,3-benzotriazin-4(3H)-one in aqueous solutions with excitation within the wavelength range 254–313 nm was studied. For both compounds, the degradation depended on the excitation wavelength: the quantum yield decreased with decreasing the excitation energy. The analysis of the irradiated solution of azinphos methyl revealed the presence of several products. 1,2,3-Benzotriazin-4(3H)-one was shown to be the major product accounting for roughly 50% of azinphos methyl conversion. Such product led in its turn to an efficient formation of anthralinic acid via the formation of an iminoketene derivative. Detailed mechanisms for the formation of the primary products from both azinphos methyl and 1,2,3-benzotriazin-4(3H)-one are proposed and discussed.