The ion trans-[Ru(OH)(py)₄(NO)]²⁺ reacts with N₃^– to give [Ru(OH)(N₃)₂(py)₂(NO)] and [Ru(N₃)₃(py)₂(NO)](py = pyridine). There was no reaction between trans-[Ru(OH)(py)₄(NO)]²⁺ and NH₂OH, nor with OH^– except at pH 13 when some replacement of pyridine occurred. The ion trans-[RuCl(py)₄(NO)]²⁺ reacted with N₃^– to give [RuCl(py)₄(N₂)]²⁺ and [RuCl(py)₄(NO)]⁺. Reduction of trans-[RuCl(py)₄(NO)]²⁺ by NH₂OH or SnCl₂ gave [RuCl(py)₄(NO)]⁺. The apparent substitution of pyridine in trans-[Ru(OH)(py)₄(NO)]²⁺ by N₃^– is ascribed to an initial reduction of the complex by N₃^–, substitution of the reduced cation, and subsequent reoxidation. At 25 °C in MeCN, trans-[Ru(OH)(py)₄(NO)]²⁺ showed an irreversible reduction wave at E_½=–0.69 V, which became reversible at –20 °C. At 25 °C trans-[RuCl(py)₄(NO)]²⁺ showed a reversible wave at E_½=–0.11 V corresponding to simple reduction to [RuCl(py)₄(NO)]⁺. The reactions of trans-[Ru(OH)(py)₄(NO)]²⁺ and trans-[RuCl(py)₄(NO)]²⁺ are compared. The structure of trans-[Ru(OH)(py)₄(NO)][PF₆]₂·H₂O was determined by X-ray diffraction: space group P2₁/n, a= 10.876(1), b= 27.711(4), c= 10.631(1)Å, β= 116.05(1)°, Z= 4, final R= 0.038 for 469 variables and 5 130 observed reflections. The ruthenium is octahedrally co-ordinated by cis-pyridines and trans-OH and NO. The Ru–OH distance is very short [1.910(3)Å] and the Ru–NO distance relatively long [1.756(3)Å]. The Ru–py distances average 2.102(3)Å.