We present a density functional theory study of a chemical reaction at a three-phase boundary. The three phases are the gas phase of the reactants and products, an adsorbate covered metallic catalyst surface phase, and an oxidized catalyst surface phase. Specifically, the CO oxidation reaction is investigated at the Pt(111)-c(4 x 2)-2CO/alpha-PtO2 phase boundary. A number of different structural phase boundary models are considered, and in every case we find special reaction sites showing smaller energy barriers than inside a reference Pt(111)-p(2 x 2)-(O + CO) coadsorption structure. Our results underline the importance of three-phase boundaries in heterogeneous catalysis. (c) 2005 Elsevier B.V. All rights reserved.