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, 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 degrees 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 O-2 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 O-2 and H2O concentrations in the feed exerted strong influence on both N2O formation and SCR activity. Decrease in O-2 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 Bronsted acidity, while H2O in higher concentrations (>5%) led to suppression of these reactions due to the coverage of active sites.