X-ray absorption fine structure (XAFS) experiments have been performed in acetonitrile solution with an in situ electrochemical Cell at controlled electric potentials On [Ru-2(mu-O) (mu-CH3COO)(2)(bpy)(2)L-2](n+) (bpy = 2,2'-bipyridine; L = pyridine (1) and 1-methylimidazole (2)) With the formal ruthenium oxidation states (RuRuIII)-Ru-II, (RuRuIII)-Ru-III, and (RuRuIV)-Ru-III Spectra were also recorded for the corresponding hydroxo-bridged Species, abbreviated as (RuRuH)-Ru-II-H-II, (RuRuH)-Ru-II-H-III, and (RuRuH)-Ru-III-H-III, which were formed by addition of p-toluenesulfonic acid to 1 and 2 and subsequent electrolysis. The X-ray absorption near edge structure (XANES) spectra all showed Similar features, although a small Shift toward higher threshold energy was observed with increasing formal ruthenium oxidation late, It is concluded from the analysis Of the EXAFS data that the ruthenium-ruthenium distance Of the mu-oxo complexes dec eases With decreasing formal ruthenium oxidation state, interpreted as resulting from decreased electrostatic repulsion between the ruthenium atoms. However, for the mu-hydroxo complexes the ruthenium-ruthenium distance decreases in the order (RuRuH)-Ru-III-H-III > (RuRuH)-Ru-II-H-II > (RuRuH)-Ru-II-H-III. The present coordination structures of the oxo- and hydroxo-bridged ruthenium dimers of the different oxidation states are compared with related iron and vanadium dimers.