Abstract A series of nitratoaquo nitrosylruthenium complexes, of general formula [RuNO(NO3)y(OH)3−y(H2O)2], has been identified. In aqueous solution they give rise to anionic and cationic ruthenium species; the higher nitrato complexes are hydrolysed at room temperature at a rapid but measurable rate to lower nitrato complexes; condensation to polynuclear species accompanies these reactions. The trinitrato complex has been prepared by the action of boiling 8M HNO3 on nitrosylruthenium hydroxide or the nitro complex Na2[RuNO(NO2)4OH] (hitherto considered a nitro complex of Ru(III)), both of which are conveniently made from ruthenium chloride. Nitro and nitrato complexes of nitrosylruthenium are also formed from ruthenium tetroxide and ruthenium alloys by the action of oxides of nitrogen or nitric acid. The stronger complexing of nitrosylruthenium by NO2− than by NO3−, as well as the degree of complexing of RuNO3+ by ligands such as F−, Cl−, point to the general similarity of these coordination complexes to those of Co(III) and Pt(IV). The nitrosylruthenium complexes are converted by sulphides to an insoluble nitrosylruthenium hydrosulphide.