N2O is a common byproduct in the NH3-SCR reaction. We analyzed the N2O formation pathways in NH3-SCR over various catalysts (Cu-ZSM-5, Fe-ZSM-5, Cu-SAPO-34, Fe-SAPO-34, Cu-SSZ-13, and Fe-SSZ-13), aided by catalyst characterization using XRD, XPS, EDS mapping, and NH3-TPD. The results showed that the NH3 nonselective catalytic reduction was the major N2O formation pathway for most of the Cu catalysts. The N2O formation at lower temperatures (<300 °C) originated mainly from decomposition of NH4NO3. In addition, NH3 nonselective oxidation was another reaction that formed N2O especially at higher temperatures. The N2O resulting from the Eley–Rideal mechanism was also favored at higher temperatures. The decomposition of NO to N2O and O2 also led to N2O formation, although its contribution was minimal. The absence of N2O yield over most Fe catalysts could be attributed to active N2O decomposition and N2O-SCR reactions. Moreover, varying O2 and H2O concentrations in the feed exerted strong influence on both N2O formation and SCR activity. Decrease in O2 level from 14% to 3% led to continual decline in N2O formation but had no effect on SCR activity until reaching a threshold concentration of 2%. H2O in lower concentrations (2–3%) facilitated N2O formation and NO conversion due to increase in Brønsted acidity, while H2O in higher concentrations (>5%) led to suppression of these reactions due to the coverage of active sites.