A selection of ruthenium and osmium nitrosyl halide complexes, MX₃(NO)(AR₃)₂[M = Ru, Os; X = Cl, Br; A = P, R = alkyl, aryl, or mixed alkyl aryl; A = As, R = phenyl] have been prepared by a convenient single-stage synthesis involving addition of ruthenium or osmium halides and N-methyl-N-nitrosotoluene-p-sulphonamide or pentyl nitrite to a solution of the appropriate tertiary phosphine or arsine in a boiling alcoholic solvent. Attempts to prepare the tri-iodo-derivatives, Ml₃(NO)(AR₃)₂, by this technique using ruthenium or osmium chlorides in the presence of a large excess of lithium iodide gave the required products, the corresponding chlorodi-iodo-derivatives or a mixture of complexes, depending on the nature of the neutral ligands present. A mechanism for the nitrosyl formation, involving ruthenium(IV) and osmium(IV) monohydrides, MX₃H(AR₃)₂, as intermediates is discussed, and an explanation for the formation of the mixed chlorodi-iodo derivatives is advanced. E.s.r. and i.r. spectroscopic evidence for the formation of a paramagnetic nitrosyl, tentatively formulated RuCl₂(NO)(PPh₃)₂, as a minor product in the synthesis of RuCl₃(NO)(PPh₃)₂ is reported.