A new route to tetrachloro-complexes of the type trans-[OsCl4L2] from mer-[OsCl3L3](L = tertiary phosphine or tertiary arsine) is described, which involves treatment with chlorine in the presence of light. The tetrachloro-osmium(III) complex trans-[OsCl4(PMe2Ph)2] reacts with tertiary phosphines, tertiary arsines, alkyl phosphites, or alkyl phosphonites (L′) to give [OsCl3(PMe2Ph)2L′]. Reduction of mer-[OsCl3L3] or [OsCl3(PMe2Ph)2L′] with either sodium borohydride or lithium aluminium hydride gives tetrahydrido-osmium complexes of types [OsH4L3] or [OsH4(PMe2Ph)2L′]. Similar reduction of trans-[OsCl4(PMe2Ph)2] gives a mixture of [OsH6(PMe2Ph)2] and [OsCl4(PMe2Ph)2]–. The tetrahydrides of type [OsH4L3] react with hydrogen chloride to give fac-[OsCl3L3]. The tetrahydrides also react slowly with EtOD in benzene to give the corresponding tetradeuterides. The hydrogen–deuterium exchange is catalysed by strong acids and is completely inhibited by a strong base such as tetrabutylammonium hydroxide. In benzene solution the high-field resonance (hydride) splittings due to coupling with phosphorus nuclei collapse immediately to a singlet resonance on adding CF3·CO2H, because of rapid exchange. Conductimetric titration shows that in the presence of a strong acid [OsH4L3] is converted into [OsH5L3]+. I.r. and n.m.r. data are given and discussed.