The relationship between metal particle structure and the reactivity and selectivity of oxide-supported metal catalysts has been explored here with well-defined Pt particles on the Zn-terminated ZnO(0001)-Zn surface, using X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), and temperature-programmed desorption (TPD). These are model catalysts for the steam reforming and selective dehydrogenation of methanol. Vapor-deposited Pt on ZnO(0001) grows in two-dimensional (2D) islands up to similar to0.7ML. The effect of the 2D island size and 3D island height are studied. Even 0.01 ML of Pt suppresses the dissociative adsorption of methanol to methoxy on Zn sites by preferential decoration of defects. Tiny amounts of Pt also affect the selectivity of methanol conversion at the Zn sites: the dehydration pathway of its Zn-bound formate intermediate is suppressed and its dehydrogenation pathway to formaldehyde is eliminated. Even Pt sites on 2D Pt islands catalyze the partial oxidation of methanol to CO and H-2. characteristic of low-index Pt facets. When the Pt islands are 2-3 layers thick. methanol decomposition is similar to that on low-index Pt faces.