Osmium halide, (CI and Br) react with monotertiary arsines Ph2RAs (R=Me, Et, Pr and Bu) in alcoholic medium to give paxamagnetic octahedrat complexes of the type OsXaL a (X = CI, Br; L = PhzRAs) which further react with carbonmonoxide to give dihalo dicarbonyl complexes of osmium(II) of th 9 type OSX2(CO)2 Lz. Similarly, osmium halides react with tertiary arsines in tim presznc, of formald 9 to give monocarbonyl complexes of osmium(H) of the type OsX2 (CO)L a. SU'uctures have been assigned to all these compounds on the basis of IR and NMR studies. Keyw~ds. Osmium ; ~trboumoaoxido ; tcxtiary argines ; IR spectra ; NMR spectra.