ASTUDY was conducted to investigate the degradation of ………chlorpyrifos, as well as studying the formation of its impurity sulfotep after storage at 54 2 oC for 1, 3, 7, 14, 21 days, at 72 2 oC for 1-5 days, and at room temperature within range 1-12 weeks of three commercial chlorpyrifos Emulsifiable Concentrate (EC) formulations (48% w/v) from the Egyptian market (manufactured from three different companies). The obtained results showed that chlorpyrifos was more stable after storage at 54 2 oC and at room temperature, whereas the rate of degradation increased after storage at 72 2 oC. There is no difference on the rate of degradation for all chlorpyrifos formulations under investigation. The sulfotep content of the three commercial formulations didn't show any correlation with storage temperature and storage time. However, in one of the commercial formulations of chlorpyrifos, the sulfotep content was more than the maximum permissible concentration of impurity recommended by FAO Specifications for chlorpyrifos (2008).