We have used a novel, high-pressure high-temperature scanning tunneling microscope, which is set up as a flow reactor, to determine simultaneously the surface structure and the reactivity of a Pt(110) model catalyst at semirealistic reaction conditions for CO oxidation. By controlled switching from a CO-rich to an O2-rich flow and vice versa, we can reversibly oxidize and reduce the platinum surface. The formation of the surface oxide has a dramatic effect on the CO2 production rate. Our results show that there is a strict one-to-one correspondence between the surface structure and the catalytic activity, and suggest a reaction mechanism which is not observed at low pressures.