The kinetics of a catalyst for hydrocarbon-selective catalytic reduction (HC-SCR) exhaust aftertreatment has been examined by means of transient experiments on a heavy-duty diesel engine rig. The influences of temperature, NO2 concentration, and the transient injection of hydrocarbon on the conversion of NO,, CO, and hydrocarbon were studied in a systematic manner. Hydrocarbon conversion was high and NO, conversion was related to the amount of injected hydrocarbon at high temperatures. At lower temperatures hydrocarbon conversion was low and NOx conversion was not directly related to hydrocarbon injection rate. Increased exhaust NO2/NO ratio resulted in NOx conversion at lower temperatures and also in accumulation of NO., on the catalyst surface. The findings are in agreement with results from recent studies of the selective catalytic reduction of NO by propene. A catalyst model was designed in accordance to these studies and fitted to results from tailored and standard European transient cycles (ETC). The model shows reasonable agreement with experimental CO, NO, and NO2) data. Experimental hydrocarbon data are not as well reproduced, presumably due to the model approximation of hydrocarbons to one species. The full catalyst model used in the study is presented, including reaction kinetics and equations for mass and heat transfer. Mechanistic aspects are discussed and related to other studies.