The selective catalytic reduction of NO_x with ethanol was investigated over a reference 3 Ag/γ-Al₂O₃ and compared to 3 Cu/γ-Al₂O₃ and 3 Fe/γ-Al₂O₃ as cost-efficient, non-noble metal catalysts. In particular, the evolution of by-products was studied using on-line Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS) analysis. For all catalysts, acetaldehyde was obtained through two different potential reaction pathways: (i) selective dehydrogenation and (ii) partial oxidation. The highest yields of acetaldehyde and NO_x-species were found for the Ag-based catalyst. In the light of the highest yield of acetaldehyde formation, exclusively for the Ag-based catalyst, a third reaction pathway including dehydration of ethanol to ethene and the oxidation to ethene oxide is possible. The adsorption capacity of NO_x and the activity for the oxidation of NO were investigated by NO_x temperature-programmed desorption (TPD) experiments. This leads to the assumption that the formation of surface nitro species over the Ag-based catalyst is pronounced as confirmed by the evolution of products from the decomposition of either nitromethane or nitroethane such as CH₄, CH₃OH, CH₂O and NH₃. The formation of NH₃ on the catalyst surface is an important reaction pathway in the EtOH-SCR. As shown by NH₃-TPD, lattice oxygen on the Fe- and Cu-containing catalysts is active for NH₃ oxidation. The resulting surface nitrates are important in the reaction mechanism of the EtOH-SCR. This was confirmed by catalytic experiments with different feed gas compositions comprising NO, NO₂ or a NO_x-free feed gas.