This investigation was carried out to study the influence of ace-tophenone, benzoin, benzil, P-nitrophenol and chlorophyl on the rate of Photodegradation of the active ingredients of pirimiphos-methyl, feni‌trothion, malathion and prothiofos insecticides which are widely used in Egypt to protect vegetable crops from economic pets. The results indicated that the percent loss of pirmiphos-methyl, fenitrothion, malathion, and prothiofos in methanol were 56.95, 16.40, 25 and 26.32% after 120 hours of exposure to UV-rays. The corre‌sponding values were 50.33, 20.53, 67.57 and, 50% for pirimiphos‌methyl; 18.03, 19.67, 41.98 and 27.16% for fenitrothion; 22.41, 19.83, 53.06 and 26.53% for malathion; and 14.47, 17.76, 58.27, 45.27 and 42.45% for prothiofos (in methanol) when exposed to UV-rays in presence of acetophenone, p-nitrophenol, benzoin, benzil and chlorophyll for the same period of exposure, respectively. The calculated residue half lives values were 98.69, 464.4, 289.13 and 272.39 hours for pirimiphos-methyl, fenitrothion, malathion and prothiofos, while these vlaues were 118.87, 361.99, 66.84, 65.82 and 120 hours for pirimi‌phos-methyl; 418.29, 379.70, 152.82, 172.43 and 262.47 hours for fenitrothion; 327.78, 376.41, 109.98, 133.05 and 269.79 hours for malathion and 532.04, 425.32, 95.17, 137.78 and 150.56 hours for prothiofos in presence of acetophenone, p-nitrophenol, benzion, benzil, and chlorophyll, respectively. Benxion and benzil compounds were found to be active as photo-sensitizers for all the studied insecticides. Chlorophyll and acetophe‌none generally showed no photosensitizing effect. Only chlorophyll showed a moderate effect with fenitrothion and prothiofas insecticides. On the contrary, p-nitrophenol had photostablizing effect on pirimiphos‌methly, malathion and prothiofos insecticides, but the effect was most evident with pirimiphos-methyl insecticide. A photostabilizing effect was clearly evident for actophenone with prothiofos insecticide irradiated by UV-light.