Near UV irradiation of aerated solutions of (Et4N)2[CuCl4] in dichloromethane causes the decomposition of CH2Cl2, as evidenced by the buildup of HCl, C2H2Cl4, and peroxides. A net reduction to [CuCl2]− occurs in the early stages, but is later reversed. In CH2Cl2, [CuCl4]2− is in equilibrium with [Cu2Cl6]2−, and only the latter species is photoactive. The decomposition is initiated by the photodissociation of chlorine atoms, which propagate to peroxy radicals, CHCl2OO. Experimental evidence, including a linear dependence of the decomposition rate on the incident light intensity and on the fraction of light absorbed by [Cu2Cl6]2−, is consistent with a mechanism in which CHCl2OO is reduced by electron transfer from [CuCl2]−, following which protonation yields CHCl2OOH. The hydroperoxide accumulates during irradiation and it too can reoxidize [CuCl2]−. The quantum yield for HCl production at the outset of irradiation at 313 nm is 1.3 mol/einstein, based on the fraction of light absorbed by [Cu2Cl6]2−.