Our experimental work aims to improve the understanding of the processes taking place in nanocluster-catalyzed reactions by systematically studying how the nanoparticle size and shape affects its chemical reactivity. For this purpose, the low temperature carbon monoxide oxidation reaction on metal-oxide-supported gold nanoparticles will be used as a model system. Different ex-situ wet chemistry methods such as nanosphere lithography or the self-assembly of metal loaded block-copolymer micelles are being used to create ordered arrays of size- and shape-selected nanospheres, nanorods and triangular nanoprism catalysts. In addition, in-situ (UHV) nanoparticle size and shape modifications by high temperature annealing, Ar+ sputtering and O2-/H2 plasma treatment are being conducted. Interconnections between directly measurable electronic phenomena and surface chemistry will be established and used to provide insight into catalytic reactions. Temperature-Programmed Desorption (TPD), Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), and X-ray Photoelectron Spectroscopy (XPS) are being employed to characterize the changes induced in the metal nanoclusters and their supports upon gas exposure. Our approach relies on the combination of ex-situ size- and shape-selected nanoparticle preparation methods and in-situ reactivity characterization measurements at different stages of well-controlled thermally and chemically induced size and shape transformations.