The photodecomposition of 1,1,1-trichloro-2,2-bis(5′-chloro-2′-methoxyphenyl)ethane (MPA) has been carried out both in solution and solid phase with exciting light > 300 nm. The major products of the solid phase photolyses are HCl and 1,1-dichloro-2,2-bis(5′-chloro-2′-methoxyphenyl)ethylene (MPE), 5,5′-chloro-2,2′-methoxybenzophenone (MPO), 1,1-dichloro-2,2-bis(5′-chloro-2′-methoxyphenyl)ethane (MPD), and 1-chloro-2,2-bis(5′-chloro-2′-methoxyphenyl)ethane (MPC). In liquid phase and in the presence of O2the photodecomposition yields many products, the major ones, in addition to HCl, being MPE, MPD, MPC, and MPO. In the absence of O2in liquid phase only HCl, MPD, MPC, and MPE were observed. In the O2free photolysis MPD and MPC are produced in larger quantity than MPE. A preliminary investigation has shown that MPE irradiated at λ > 300 nm also photodecomposes. A check carried out by the Canada Department of Agriculture indicates that MPA is a pesticide although it may not be as effective as DDT.