Since Cu is known to decorate Ru surfaces but is not active for ethane hydrogenolysis, it provides a means to study the effect of modifier decoration on this highly structure sensitive reaction. The effect of this decoration on the concentration of the active intermediates and the surface kinetics during ethane hydrogenolysis on Ru/SiO2 has been studied using isotopic transient kinetics analysis. The abundances, coverages, and lifetimes of active surface intermediates of the reaction were measured under initial reaction conditions. Approximate intrinsic activities and abundances of surface dicarbon and monocarbon species were found to change significantly with the addition of Cu. The results show that low loadings of Cu appeared to block preferentially the sites which were the most active but much less abundant on the Ru surface, confirming the theoretical calculations of Strohl and King [(J. Catal. 116, 540 (1989)] which suggested a nonuniform distribution of Cu on Ru crystallites. Ensemble sizes for ethane adsorption and hydrogenolysis were found to be ca. 5 and 12, respectively, based on a correlation between the change in surface abundance of each reactive intermediate species and the fraction of Ru surface atoms exposed. Approximate intrinsic activity related to breakage of the C-C bond during ethane hydrogenolysis varied significantly for different sites, while that related to hydrogenation of monocarbon species was essentially the same for all sites.