The mechanism of the reduction of NO by C3H6 under lean-burn conditions on a Pt catalyst has been investigated using transient techniques. Switches of O-2 and C3H6 into and out of a stream of C3H6/NO/O-2 over a 1% Pt-SiO2 catalyst were performed at temperatures above and below that at which maximum NOx conversion is seen, i.e., 220 and 250 degrees C. The transient reaction studies show that at low temperatures the Pt is reduced and covered with carbonaceous material while at high temperatures it is oxidised and covered with O-ads species. Rapid changes in the surface oxidation state of the Pt are observed after the removal or reintroduction of the O-2 and C3H6 reactants at all temperatures. The results of each individual switch are discussed in terms of a mechanism for NOx reduction and alkene oxidation over Pt-based catalysts in the "lean-NOx" process. The conclusions are found to be consistent with our previous model for the NOx reduction model. It is proposed that at low temperatures the NO reacts by decomposition on reduced Pt sites, whereas at high temperatures it reacts with O-ads on oxidised Pt to form NO2.