This paper describes a reaction-reactor model, utilizing alternative reaction pathways, developed to explain experimental results from the total oxidation of ethylene on a Pt/SiO2 catalyst. Transient temperature-programmed reaction and concentration-programmed reaction experiments were utilized to study the reaction through FTIR spectroscopic and surface temperature measurements combined with bulk measurements of concentration and temperature. Different elementarystep pathways were incorporated into the concentration and energy balances in performing the simulations. One pathway utilized adsorbed CO, formed from adsorbed ethylene, as the route to CO2 formation. This pathway is shown to become important under ethylene-rich conditions. The second pathway utilized the direct reaction between adsorbed ethylene and adsorbed oxygen as the path to CO2 formation. The latter route is shown to become important under oxygen-rich conditions. Finally, an example is utilized to show the advantages of the nonequilibrium elementary-step modeling approach over conventional “equilibrium” models where only one step is taken to be rate-determining.