The complex [Ru(terpy)(tmen)(OH2)]2+(terpy = 2,2′:6′2″-terpyridine, tmen =N,N,N′,N′-tetra-methylethylenediamine) and its corresponding oxo complex have been prepared. Their spectral properties and redox properties in aqueous solution were investigated. Evidence for two one-electron redox processes has been obtained for the RuIV–RuIII and RuIII–RuII couples. The reactions of the new ruthenium(IV) oxo species with olefins have been found to proceed with second-order kinetics, rate =k2[RuIV][olefin] where k2=(3.3 ± 0.3)× l0–3 and (2.0 ± 0.2)× 10–2 dm3 mol–1 s–1 at 25 °C for norbornene and styrene respectively.