Hexachloroosmate(IV) effectively catalyzes the photodecomposition of chloroform in aerated solutions. The decomposition products are consistent with a mechanism in which excited state OsCl(6)(2-) reduces chloroform, rather than one involving photodissociation of chlorine atoms. Trace amounts of ethanol or water in the chloroform lead to photosubstitution to form OsCl(5)(EtOH)(-) or OsCl(5)(H(2)O)(-), neither of which is photocatalytically active.