The anodic oxidation of Cp*Ru(eta(5)-C6H9O) 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(CH3CN)(3)](+) PF6-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)(eta(3)-C6H9O) was obtained in 70% yield by oxidative addition of molecular oxygen. A slow chemical reaction between [Cp*Ru(CH3CN)(3)]+PF6- and Cp*Ru(O-2)(eta(3)-C6H9O) leads to the activation of a Cp* ligand giving an aldehyde derivative (eta(5)-C5Me4CHO)Ru(eta(5)-C6H9O).