Solid Ru/CeO₂ catalysts, which are effective for the catalysis of various highly atom-efficient organic transformations such as dehydrogenative synthesis of indole, regio- and stereo-selective coupling of unsaturated hydrocarbons, C–C bond-forming reactions via the activation of aromatic C–H bonds, and selective addition of carboxylic acids to alkynes, have been developed. For these reactions, CeO₂-supported and, in some cases, ZrO₂-supported catalysts show excellent activities, whereas ruthenium catalysts on other oxide supports are not effective. Spectroscopic studies have revealed that various types of catalytically active, low-valent ruthenium species are generated from ruthenium(IV) oxo species that are exclusively formed on the surface of CeO₂ under appropriate conditions. These solid ruthenium catalysts can be recycled without loss of activity or significant leaching of ruthenium species into the solution, and these features, as well as the wide range of applicable reactions, make the present catalytic system quite attractive from both practical and environmental perspectives.