To analyze the effect of the support crystallite size on the activity of gold catalysts for CO oxidation, gold was supported on rutile with crystallite sizes between 10 and 76 nm. The catalysts were characterized by X-ray powder diffraction, refinement of the crystalline structures, transmission electron microscopy, and nitrogen adsorption and were tested for the oxidation of CO at temperatures below 200 degrees C. Rietveld refinement provided the crystallite dimensions of the support, its morphology, its percentage of microstrain, and its specific area, which was similar to that obtained from nitrogen adsorption experiments. This specific area together with the microstrain determined the size of the gold crystallites: As these two parameter decreased with increasing support crystallite size, the gold crystallite dimensions increased. The specific area of the gold crystallites varied between 54.7 m(2)/g for the largest crystallites to 108.2 m(2)/g for the smallest crystallites, and the respective conversion of CO into CO2 at 50 degrees C changed from 1 to 99. Therefore, the change in surface area alone cannot explain the changes in the catalytic activity. The gold crystallite morphology corresponded to cubooctahedra with extra layers of atoms on the surface parallel to the (111) planes. The smallest gold crystallite was modeled with the gold atoms treated as hard spheres; it contained a total of 893 gold atoms, 468 of them on the surface, with coordination numbers of 5 and 7-11.