The title compound, [Os₃(CO)₉(NO)₂], has been prepared by the reaction of NO with [Os₃(CO)₁₂] in n-octane at 126 °C. Carbon-13 n.m.r. spectroscopy indicates that the structure is derived from that of [Os₃(CO)₁₂] by the replacement of three CO ligands about one osmium by two terminally bound NO ligands. The stereodynamic behaviour of the molecule, as determined by ¹³C n.m.r. spectroscopy over a range of temperature, has been interpreted in terms of a process involving terminal–bridging NO interchange of the type previously observed for certain CO-cluster compounds. On reaction with ligands L (= NH₃, NEtH₂, and CO) the formation of the unstable adducts [Os₃(CO)₉(NO)₂L] is observed; with L = P(OMe)₃ or PPh₃ the substituted complexes [Os₃(CO)₈(NO)₂{P(OMe)₃}] or [Os₃(CO)₉(NO)₂(PPh₃)] are produced. A single-crystal analysis of the trimethyl phosphite compound confirms that two terminally bound NO ligands are present. For L = pyridine (py), [Os₃(CO)₉(NO)₂] forms the dinitrosyl-bridged compound [Os₃(CO)₉(µ₂-NO)₂(py)], quantitatively. The factors which govern the course of these reactions are discussed.