The work presented inhere concerns the study of the photochemical behavior of the organophosphorus azinphos-methyl (AZM) and its model molecule, benzotriazone. For both compounds, the photodegradation efficiency increases with the excitation energy. Such effect can be attributed to a difference in the excited states reactivity or to homolytic bond dissociations of the triazone moiety. The degradation of azinphos-methyl involving direct excitation leads to the formation of benzotriazone which photodegrades in its turn into anthranillic acid. In the case of cyanophos, another organophosphorus pesticide, the degradation involves both singlet and triplet excited state. Several chemical processes were present: homolytic cleavage, hydrolysis. 4-cyanophenol, cyanophos-oxon and [(dimethoxyphosphorothiolyl) oxy] benzoïc acid represent the main observed photoproducts. The study of azinphos-methyl degradation upon direct irradiation has been also studied at the surface of clays (kaolinite, bentonite) and iron oxide (goethite). Adsorption of the pesticide on clays leads to an enhancement of its spectroscopic features permitting a better solar light absorption. Moreover, the photodegradation of AZM appears to be efficient, in Suntest device, on all the studied clays. The efficiency of the reaction depend on the type of clay, thickness, water content, and iron(III) concentration. The degradation of azinphos-methyl involves both direct photochemical reactions and clay photoinduced processes. The involvement of hydroxyl radical was demonstrated by ESR and fluorescence measurements. Among several other products, azinphos-methyl oxon was found to be formed and to accumulate in the medium inducing an increase of the toxicity.