Most of the attempts to quantify the reactivity and selectivity of catalytic systems have been focused on characterization of specific surface structures, where electronic perturbations are induced by varying the particle size or by adding small amounts of substances. Addressing specific systems the lecture will illustrate the important contribution of chemical specific imaging combined with laterally resolved structural characterizations to understanding key factors controlling the processes at complex surfaces and interfaces related to catalysis. The local composition and reactivity of the phases that can be formed and coexist during oxidation reactions will be described comparing the results obtained with single crystal metal surfaces, micrometer-sized metal particles and nano-crystalline films supported on different substrates. The complexity of the realistic reaction systems, where mass transport processes can introduce lateral heterogeneity in the composition and structure of the interface creating local micro-reactors with different catalytic activity, will be discussed on the bases of recent results with bimetallic systems. The effect of electron confinement will be shown for oxidation of ultrathin metallic films, where varying the film thickness the sensible differences in the local oxidation rate are observed and correlated to periodic changes in the density of electronic states induced by quantum-well states crossing the Fermi level. 1. P. Dudin et al, J. Chem. Phys.B 109, 2005, 13649. 2. A. Locatelli et al, J. Am. Chem. Soc. 127, 2005, 2351. 3. L. Aballe et al, Phys. Rev. Lett. 93, 2004, 196103.