The structures of the RuOx species supported on monoclinic ZrO2 (m-ZrO2) after treatment in a wide temperature range 673-1323 K were characterized by X-ray diffraction, Raman, infrared and X-ray photoelectron spectroscopies, and temperature-programmed reduction in H-2. Their catalytic properties were examined in the selective oxidation of methanol to methyl formate at 373 K. The RuOx species were present as highly dispersed RuO42- on m-ZrO2 with Ru surface densities of 0.2-2.2 Ru/nm(2), irrespective of the treatment temperatures. Their molecular structures evolved from umbrella-like dioxo (O=)(2)Ru(OH)-O-Zr (containing a Ru-OH bond and a Ru-O-Zr bond to m-ZrO2 surface) into tetrahedral dioxo (O=)(2)Ru-(O-Zr)(2) with increasing the treatment temperatures from 673 to 773 K, and into pyramidic mono-oxo O=Ru-(O-Zr)(4) above 1173 K, induced by the stronger interaction between the RuOx species and underlying m-ZrO2 surface at the higher temperatures. Following such structural changes, the turnover rates of the methanol oxidation increased markedly, and reached the greatest value (39.0 mol/mol Ru-h) on the catalyst treated at 1223 K, as a result of the parallel increase in the reducibility of the RuOx species, consistent with the known Mars-van Krevelen redox mechanism using lattice oxygen atoms on RuOx. These understandings may be useful for improving the reactivity of the RuOx-based catalysts for the selective oxidation of methanol as well as of other alcohols and probably light hydrocarbons, for instance, by synthesis and tuning of the more reducible di-oxo RuO42- structures. (C) 2011 Elsevier B. V. All rights reserved.