Abstract The photochemical reduction of azoxybenzene to azobenzene has been investigated. The direct irradiation of azoxybenzene leads to a rearrangement to 2-hydroxyazobenzene and to cis-trans isomerization, whereas the sensitized irradiation leads to a reduction to azobenzene in the presence of a sensitizer with a triplet excitation energy greater than ca. 62 kcal/mol. 2-Hydroxyazobenzene and cis-azoxybenzene seem to be formed from the excited singlet state of azoxybenzene, and azobenzene, from the excited triplet state. Even without a sensitizer, the irradiation of benzo[c]cinnoline N-oxide, which is incapable of undergoing rearrangement or isomerization, brings about a photochemical reduction, which is affected by a sensitizer or a quencher, and also by the hydrogen-donating ability of the solvent used (isopropanol>ethanol>cumene>toluene). Acetone is yielded when isopropanol is used as the solvent. On the basis of these results, a mechanism in which benzo[c]cinnoline is formed from the excited triplet state of benzo[c]cinnoline N-oxide, followed by hydrogen abstraction from a solvent, is proposed.