The steady-state catalytic reduction of NO with CO on iron oxide was described with three different models, which all fit the experimental data equally well. By applying the transient method to this reaction, it has been shown that for reduced catalysts without adsorbed CO, the N2O reduction step is well described qualitatively by two of the models, whereas only one of them can describe the NO reduction step. In the presence of adsorbed CO, however, none of these models can explain the observed transient behaviour. Therefore, a modification of the best model was proposed and tested via computer simulation, which showed good agreement with transient experiments.