Catalytic reduction of SO2 to elemental sulfur was carried out over supported ruthenium catalysts that were derived not only from hexaruthenium carbonyl cluster complexes but also from RuCl3 as the precursor. The activity was largely dependent on the kind of metal oxide support used. Compared to the known systems, the TiO2-supported ruthenium catalysts operated with higher efficiency at lower temperature. Moreover, the selectivity was totally to elemental sulfur, forming no detectable amount of unfavorable H2S. H2S-free catalysis appeared to be a common feature of ruthenium catalysts regardless of the kind of precursors. The TiO2-supported catalyst derived from [N(PPh3)(2)](2)[Ru6C(CO)(16)] was much more active within the 463-508 K temperature region than the catalyst conventionally prepared from RuCl3, while both showed similar activity at more elevated temperatures. The kind of cation in anionic cluster complexes and the presence of an interstitial atom are important factors in generating supported catalysts that operate under mild conditions.