Cobalt-exchanged ferrierite zeolite catalysts were found to be very active (about twice the activity compared to Co-ZSM-5) for the selective reduction of NO(x) with CH4. The presence of oxygen (gaseous O2 or O atoms via N2O decomposition) greatly enhanced the NO(x) conversion to N2. In the absence of O2, NO2 can be effectively reduced with CH4, but its conversion decreased with increasing temperature at T > 500-degrees-C with a formation of small amount of O2. Temperature programmed desorption measurements show that the presence of O2 alters the NO adsorption/desorption on Co-ferrierite, yielding an additional, high-temperature NO desorption peak. Partial pressure dependencies were measured as a function of reaction temperature under both dry and wet conditions. In the absence of added H2O vapor, the empirical reaction order with respect to NO (0.2 to 0.4) increased with increasing temperature with no change in the reaction order for CH4 (0.7). Addition of H2O vapor (2%) significantly increased the empirical reaction orders with respect to both NO (0.7) and CH4 (0.8-0.9). Under wet conditions, the reaction orders are higher at low temperatures. The added H2O vapor also substantially raised the apparent activation energies for both NO reduction and CH4 combustion. The turnover frequencies for both reactions increased with increasing CO2+ level, demonstrating the heterogeneity of the Co2+ sites. The differences in activity and selectivity and in the distribution of active sites between Co-ferrierite and Co-ZSM-5 may be related to the different channel systems of these two zeolites.