Aiming at a more detailed understanding of support effects on the catalytic performance of supported Ru catalysts, here specifically in the selective methanation of CO, we have prepared and investigated a set of three isostructural Ru catalysts with different supports (TiO2, Al2O3, and ZrO2), but similar surface area, Ru loading, and Ru particle size. The activity was found to increase with increasing reducibility in the order Ru/Al2O3 <= Ru/ZrO2 < Ru/TiO2, which goes along with a significant increase of electron density on the Ru NPs, in the number of O-vacancies, and the amount of basic sites in the oxide supports, as probed by XPS and pyrrole titration. In situ FTIR spectra recorded during reaction indicate that the high activity on Ru/TiO2 is correlated with the highest COad/Ru-multicarbonyl coverage, while Ru/ZrO2 and Ru/Al2O3 exhibit significantly lower COad coverages, indicative of lower CO adsorption energies. Kinetic measurements using isotope labelled gas mixtures, H-2/CO and D-2/CO, show a small, but distinct inverse kinetic isotope effect (KIE), which is more pronounced on Ru/TiO2 (R-H/R-D = 0.76) than on Ru/ZrO2 and Ru/Al2O3 ((R-H/R-D = 0.91). These trends are explained by and discussed in terms of differences in electronic metal-support interactions (EMSIs), caused by the differences in O-vacancy formation (reducibility) of the support materials and consequently in charge transfer to the Ru nanoparticles. Such effects are considered to be of general importance for reactions taking place under strongly reductive conditions. (C) 2019 Elsevier Inc. All rights reserved.