Chlorpyrifos was reported to undergo indirect (chemical decomposition or change as a result of absorption of light by natural substance that then react with the chemical of interest) by and direct (chemical decomposition or change initiated by the absorption of light by the chemical of interest) photolysis [t1/2 = 2 h (indirect) and 5 h (direct)]. The result obtained for indirect photolysis is consistent with the Estimation Program Interface (EPI) Suite estimations. This study confirms the formation of chlorpyrifos-oxon via photolysis. Chlorpyrifos-oxon was reported to undergo indirect and direct photolysis [t1/2 = 8 h (indirect) and 6 h (direct)]. The EPI Suite estimated indirect photolysis was similar to the calculated value