The chemical changes that occurred in two copper-based catalysts (Cu-ZSM-5 and Cu-Al2O3) during the selective reduction of NO with propene in the presence of oxygen were studied using in situ X-ray absorption near edge structure (XANES). For the quantitative analysis of the XANES spectra, a mathematical procedure based on evolving factor analysis (EFA) has been applied. Correlation with catalytic data shows that copper is fully oxidized in both systems when the conversion of propene is complete and the conversion of NO to N-2 reaches its maximum value. The XANES analysis together with comparison of the catalytic behavior of Cu-ZSM-5 with Cu-Al2O3 and Cu-SiO2 systems indicates that the rate limiting step of the reaction takes place on cupic oxides (although other species may also participate). This conclusion is of general significance because it establishes the physical basis for the role of Cu in selective catalytic reduction of NO by hydrocarbons; this explains the recently recognized fact that zeolitic supports do not play an essential role in the reaction and that the mechanism may not involve a Cu2+/Cu1+ redox cycle or, if it does, the cycle is dominated by the oxidized species. In situ XANES analysis coupled with EFA is demonstrated to be a useful technique to obtain information about the active sites present on a catalyst during reaction.