Rate expressions for the oxidation of carbon monoxide and hydrogen over a Pt catalyst are achieved using a combination of surface modeling and experimental measurements. The conditions studied are those expected within a preferential oxidation (PrOx) reactor in a fuel processor for a fuel cell stack. Convenient rate expressions for subsequent reactor modeling in a companion paper are derived from a set of elementary rate expressions using the approximation of near saturation of the catalyst surface by carbon monoxide and near partial equilibrium of adsorption/desorption of carbon monoxide and hydrogen. These rate expressions require only two Arrhenius kinetic parameters and satisfactorily represent our measured rates over the nearly three orders of magnitude variation in carbon monoxide and oxygen concentrations between 80 and 200 degrees C. (c) 2005 Elsevier Ltd. All rights reserved.