A transient kinetic model was developed for the CO oxidation by O-2 over a Pt/Rh/CeO2/gamma-AL(2)O(3) three-way catalyst. The experiments which were modelled consisted of periodically switching between a feed stream containing 0.5 mol% CO in helium and a feed stream containing 0.5 mol% O-2 in helium, with a frequency from 0.1 to 0.25 Hz, in the temperature range 393-433 K. These temperatures are representative for cold start conditions. The transient experiments yield information about the reaction mechanism. A transient kinetic model based on elementary reaction steps was developed which describes the experimental data in the above mentioned range of experimental conditions adequately. The kinetic model consists of two monofunctional and one bifunctional contribution. The first monofunctional reaction path comprises competitive adsorption of CO and O-2 on the noble metal surface followed by a surface reaction. The second monofunctional reaction path consists of CO adsorption on an oxygen atom adsorbed on the noble metal surface, followed by a reaction to CO2. The bifunctional reaction path involves a reaction between CO adsorbed on the noble metal surface and oxygen from ceria at the noble metal/ceria interface. Also, reversible adsorption of carbon dioxide on the support is taken into account. The kinetic parameters, i.e. preexponential factors and activation energies for the different elementary reaction steps, and the oxygen storage capacity were estimated using multi-response non-linear regression analysis of the oxygen, carbon monoxide and carbon dioxide outlet concentrations.