The reaction paths of the selective catalytic reduction of NO2 by C2H4 over H-ferrierite were investigated by identifying and measuring the reactivity of the by-products under various coexisting gas conditions. Nitroethylene (NE), HCN, and HNCO were detected as nitrogen-containing by-products. It was found that there were two reaction pathways for N-2 formation: the direct reaction between NE and NO2, and NE decomposition followed by the formation and hydrolysis of HNCO. The latter pathway led to the formation of NH3, which further reacted with NOx to form N-2.