Channel electrode methodology is used to study the photoelectrocatalytic reduction of 4-chlorobiphenyl (CBP) in acetonitrile solution using 9,10-diphenylanthracene (DPA) as a mediator. The latter is reduced at a lower potential than is required for the direct reduction of the substrate, and the resulting radical anion, DPA(.-), when photoexcited irreversibly transfers an electron to CBP. The mechanism of the reaction is explored, and it is shown that in addition to the catalytic reduction of the biphenyl by excited state DPA(.-) the latter also undergoes quenching by DPA and photoinduced disproportionation leading to the dianion, DPA(2-), which undergoes irreversible chemical transformation. Kinetic parameters are reported.