In order to elucidate electronic effects on the oxidation of CO on small Ru clusters, we investigated this reaction on well defined Ru/Au(111) model systems via parallel in-situ STM studies of the structure and electrochemical deposition of Ru on Au(111) in H2SO4 solution and cyclic voltammetry of CO monolayer oxidation on these surfaces. The Ru deposit consists of nanoscale islands, which coalesce with increasing coverage. The Ru saturation coverage depends on the deposition potential, resulting in Ru submonolayer (> 0.1 V), (defective) monolayer (greater than or equal to - 0.1 V), and multilayer films (< -0.1 V). At potentials > 0.6 V irreversible formation of Ru oxide/hydroxide species is observed, which can be partly reduced in the range 0.4 to 0.0 V. CO stripping commences at approximate to0.1 V and occurs over a broad potential range. From the stripping charge a local CO coverage on the Ru monolayer islands of 0.7 ML was estimated. The observed influence of the morphology of the Ru deposit on the CO stripping voltammetry is explained by (local) variations in the CO adsorption energy due to electronic modifications of the Ru film.