In aqueous acidic solutions trans-[RuVI(L)(O)2]2+ (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)]2+. When ≤1 mol equiv NO is used, the intermediate RuIV species, trans-[RuIV(L)(O)(OH2)]2+, can be detected. The reaction of [RuVI(L)(O)2]2+ with NO is first order with respect to [RuVI] and [NO], k2=(4.13±0.21)×101 M−1 s−1 at 298.0 K. ΔH≠ and ΔS≠ are (12.0±0.3) kcal mol−1 and −(11±1) cal mol−1 K−1, respectively. In CH3CN, ΔH≠ and ΔS≠ have the same values as in H2O; this suggests that the mechanism is the same in both solvents. In CH3CN, the reaction of [RuVI(L)(O)2]2+ with NO produces a blue-green species with λmax at approximately 650 nm, which is characteristic of N2O3. N2O3 is formed by coupling of NO2 with excess NO; it is relatively stable in CH3CN, but undergoes rapid hydrolysis in H2O. A mechanism that involves oxygen atom transfer from [RuVI(L)(O)2]2+ to NO to produce NO2 is proposed. The kinetics of the reaction of [RuIV(L)(O)(OH2)]2+ with NO has also been investigated. In this case, the data are consistent with initial one-electron O− transfer from RuIV to NO to produce the nitrito species [RuIII(L)(ONO)(OH2)]2+ (k2>106 M−1 s−1), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)]2+ and NO2− (k2=54.7 M−1 s−1).