A detailed and comparative mechanistic study of the photoelectrochemical dehalogenation of the four p-halonitrobenzenes, X-C6H 4-NO2 (X = F, Cl, Br, or I) in acetonitrile solution is reported. In the case of p-iodonitrobenzene, iodide loss is accomplished by electrochemical reduction alone. The formation of l- is shown to take place via an ECE process with the ultimate generation of the radical anion of nitrobenzene. Dual photo- and electro-chemical activation of p-chloronitrobenzene and p-br...