The anodic oxidation of Cp*Ru(η 5 -C 6 H 9 O) was performed in acetonitrile on a glassy carbon electrode. The electrochemical behavior is strongly dependent on the presence of both argon and oxygen in acetonitrile solutions. Under an argon atmosphere an acetonitrile coordinated compound [Cp*Ru(CH 3 CN) 3 ] + PF 6 - was obtained. This compound is generated following a mechanism involving cleavage of the mesityl oxide ligand. In contrast, under an oxygen atmosphere, a novel peroxo compound Cp*Ru(O 2 )(η 3 -C 6 H 9 O) was obtained in 70% yield by oxidative addition of molecular oxygen. A slow chemical reaction between [Cp*Ru(CH 3 CN) 3 ] + PF 6 - and Cp*Ru(O 2 )(η 3 -C 6 H 9 O) leads to the activation of a Cp* ligand giving an aldehyde derivative (η 5 -C 5 Me 4 CHO)Ru(η 5 -C 6 H 9 O).