Ruthenium μ-oxo dimers with chloro-substituted 2,2′-bipyridines [L 2 (H 2 O)Ru III ORu III (OH 2 )L 2 ] 4+ (L= 4,4′-dichloro- or 5,5′-dichloro-2,2′-bipyridine) were synthesized. Electrochemical studies on the complexes showed that the Ru III |O|Ru III dimer undergoes a reversible one-electron oxidation to Ru III |O|Ru IV followed by a reversible multi-electron oxidation assignable as the oxidation of Ru III |O|Ru IV to Ru IV |O|Ru V . The Ru III |O|Ru IV and Ru IV |O|R V species were characterized by thin-layer spectroelectrochemistry. The Ru IV |O|Ru V dimer is an active catalyst for the chemical or electrochemical oxidation of water to dioxygen. Upon reduction, the Ru III |O|Ru III dimer undergoes reductive cleavage to give [Ru II L 2 (H 2 O) 2 ] 2+ . The chloro-substituted ruthenium dimers are more stable towards reductive cleavage in mildly acidic media than the unsubstituted [(bpy) 2 (H 2 O)Ru III ORu III (OH 2 )(bpy) 2 ] 4+ (bpy= 2,2′-bipyridine).