This investigation was carried out to study the influence of ben-win, benzil, P-nitrophenol and 8-hydroxyquinoline on the rate of photo-degradation of the active ingredients of diazinon and pirimiphos-methyl insecticides which are widely used in Egypt to protect vegetable crops from economic pests. The calculated residue half lives values with UV light exposure were 13.59, 12, 24, 3.53 and 3.47 hours for diazinon alone & in pres‌ence of p-nitrophenol, 8-hydroxyquinoline, benzil & benzion, respective‌ly. While these values were 8.77, 7.00, 19.80 . 0.81 and 0.82 hours for the same treatments, respectively. We conclude that the 8-hydroxy quinoline was more stabilizing agent than 4-nitrophenol. Benzil was found to be more sensitizing agent than benzoin. While 4-nitrophenol has almost the same effect that occurred when pirimiphos-methyl was applied alone, but 8-hydroxyquinoline stabilized in the insecticide by its powerful stabilizing action. The calculated residue half life values with sun light exposure of diazinon and pirimiphos-methyl were 1.01 and 0.59 hours, while the val‌ues were 0.67, 0.6, 0.29 and 0.53 hours for diazinon and 0.42, 1.02, 0.21 and 0.29 hours in the presence of p-nitrophenol, 8-hydroxy quino‌line benzil and benzion, respectively. The stabilizing agent in this experi‌ment had no effect, while the sensitizing agent had the expected effect from the beginning of the experiment, including that benzil was a more powerful sensitizing agent than benzion .