Experimental data from the UV peroxidation of bis(2-chloroethyl) ether (DCEE) in aqueous solution indicated the rate equation is 0.163[DCEE](0.61)[H2O2](0.54). Important intermediates identified are 2-chloroethyl acetate, an enolic tautomer of 8-chloroethyl acetate, 2-chloroethoxyethene, 2-chloroethanol, acetaldehyde, ethylene oxide, and chloroethene. All the intermediates could be reduced to undetectable levels after 30 min of-irradiation when DCEE/H2O2 initial ratio of 1/10 was used. A reaction mechanism with complex pathways through both the attack of hydroxy free radical and the direct photolysis on DCEE was proposed. Intermediate identification and the rate equation suggested that the pathways in which DCEE is attacked by hydroxy free radicals are predominant. The rate equation derived from this mechanism predicted the peroxidation of DCEE is half-order with respect to both DCEE and H2O2 concentrations.