The pre-treatment of highly active and selective Ru/zeolite catalysts for the selective CO methanation in CO2-rich H-2 atmospheres involves a calcination step before the catalyst activation to reach their high activity and selectivity. In this study, we investigated the impact of the calcination procedure on a 2.2 wt.% Ru/zeolite catalyst by evaluating the resulting changes in the catalyst structure and reaction characteristics by combined kinetic, in situ extended X-ray absorption fine structure (EXAFS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. Calcination in synthetic air, followed by the standard activation procedure in reaction gas, results in a decrease in mean Ru particle size with increasing calcination temperature, from similar to 1.5 nm to similar to 1.0 nm. The increase in dispersion goes along with a distinct increase in selectivity of the Ru/zeolite catalyst for CO methanation at extremely low concentrations of CO (100 ppm). Possible mechanisms and the physical origin responsible for these changes in catalyst structure and reaction characteristics are discussed. (C) 2012 Elsevier Inc. All rights reserved.