Reaction of trans-[Ru-VI(L)(O)(2)](2+) (1, L) 1,12-dimethyl-3,4: 9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane, a tetradentate macrocyclic ligand with N2O2 donor atoms) with nitrite in aqueous solution or in H2O/CH3CN produces the corresponding ( nitrato) oxoruthenium(IV) species, trans-[RuIV(L)(O)(ONO2)](+) (2), which then undergoes relatively slow aquation to give trans-[Ru-IV(L)(O)(OH2)](2+). These processes have been monitored by both ESI/MS and UV/vis spectrophotometry. The structure of trans- [ RuIV( L)( O)( ONO2)]+ ( 2) has been determined by X-ray crystallography. The ruthenium center adopts a distorted octahedral geometry with the oxo and the nitrato ligands trans to each other. The Ru=O distance is 1.735(3) angstrom, the Ru-ONO2 distance is 2.163(4) angstrom, and the Ru-O-NO2 angle is 138.46(35). Reaction of trans-[RuVI(L)( O-18)(2)](2+) (1-O-18(2)) with (NO2-)-O-16 in H2O/CH3CN produces the O-18-enriched (nitrato) oxoruthenium( IV) species 2-O-18(2). Analysis of the ESI/MS spectrum of 2-O-18(2) suggests that scrambling of the O-18 atoms has occurred. A mechanism that involves linkage isomerization of the nitrato ligand and reversible oxygen atom transfer is proposed.