We report an investigation of the reduction of nitric oxide by carbon monoxide over a Pt(1 0 0) model catalyst at atmospheric pressure (1.25 bar). A combination has been used of high-pressure scanning tunneling microscopy and simultaneous mass spectrometry to correlate observations of the surface structure with the reaction rate and reaction mechanism. The STM images suggest that depending on the precise composition of the reactant gas mixture the Pt(1 0 0) surface switches between the quasi-hexagonal structure, characteristic for this surface in vacuum, and the bulklike (1 x 1) structure that is 20% less dense. The reaction rates that we observe are interpreted in the framework of classical Langmuir-Hinshelwood kinetics on both surface structures. (C) 2010 Elsevier B.V. All rights reserved.