The effects of RuOx structure on the selective oxidation of methanol to methyl formate (MF) at low temperatures were examined on ZrO2-supported RuOx catalysts with a range of Ru surface densities (0.2-3.8 Ru/nm(2)). Their structure was characterized using complementary methods (X-ray diffraction, Raman and X-ray photoelectron spectra, and reduction dynamics). The structure and reactivity of RuOx species change markedly with Ru surface density. RuOx existed preferentially as RuO42- species below 0.4 Ru/nm(2), probably as isolated Zr(RuO4)(2) interacting with ZrO2 surfaces. At higher surface densities, highly dispersed RuO2 domains coexisted with RuO4(2-) and ultimately formed small clusters and became the prevalent form of RuOx above 1.9 Ru/nm(2). CH3OH oxidation rates per Ru atom and per exposed Ru atom (turnover rates) decreased with increasing Ru surface density. This behavior reflects a decrease in intrinsic reactivity as RuOx evolved from RuO42- to RuO2, a conclusion confirmed by transient anaerobic reactions of CH3OH and by an excellent correlation between reaction rates and the number of RuO42- species in RuOx/ZrO2 catalysts. The high intrinsic reactivity of RuO42- structures reflects their higher reducibility, which favors the reduction process required for the kinetically relevant C-H bond activation step in redox cycles using lattice oxygen atoms involved in CH3OH oxidation catalysis. These more reactive RuO42- species and the more exposed ZrO2 surfaces on samples with low Ru surface density led to high MF selectivities (e.g. similar to 96% at 0.2 Ru/nm(2)). These findings provide guidance for the design of more effective catalysts for the oxidation of alkanes, alkenes, and alcohols by the synthesis of denser Zr(RuO4)(2) monolayers on ZrO2 and other high surface area supports.