The preparation of the series of osmium(II) complexes, tram- and cis-[Os(pdma)₂X(CO)][BPh₄][X = Cl, Br, or I; pdma =o-phenylenebis(dimethylarsine)] and [Os(pdma)₂X₂(CO)](X = Br or I), is reported, together with their i.r. and n.m.r. spectra. The complexes [Os(pdma)₂X₂(CO)] contain one uni- and one bi-dentate pdma ligand, and a crystal structure analysis of [Os(pdma)₂Br₂(CO)] shows that the osmium atom is octahedrally co-ordinated by two trans-Br ligands, one CO trans to a bidentate pdma, and the arsenic of one unidentate pdma. The unco-ordinated arsenic of the unidentate pdma is 4.36 Å from the osmium. Crystal data: a= 15.70(1), b= 11.48(1), c= 17.51(1)Å, β= 115.89(3)°, Z= 4, space group P2₁/c: final R 0.048 for 2 047 observed reflections (by full-matrix refinement). The [Os(pdma)₂X₂(CO)] complexes give cis-[Os(pdma)₂X(CO)]⁺ on heating, and the cis complexes isomerize to the trans on further heating. The preparation and properties of the osmium(III) complexes [NBuⁿ₄][Os(pdma)X₄(CO)](X = Cl, Br, or I), which also contain a unidentate pdma ligand, are reported.