Hexachloroosmate(IV) effectively catalyzes the photodecomposition of chloroform in aerated solutions. The decomposition products are consistent with a mechanism in which excited state OsCl62− reduces chloroform, rather than one involving photodissociation of chlorine atoms. Trace amounts of ethanol or water in the chloroform lead to photosubstitution to form OsCl5(EtOH)− or OsCl5(H2O)−, neither of which is photocatalytically active.