Selective reduction of NO by CH4 in the presence of excess O-2 has been studied on cobalt-ion-exchanged synthetic clinoptilolite (Co-CLI) zeolite. The catalytic results are compared with those obtained from Co-ZSM-5 and Co-FER, the two most widely studied catalysts for this reaction. At T greater than or equal to 500 degrees C, Co-CLI is much more active for NO reduction than Co-ZSM-5, but it is less active than Co-FER. However, it is found that the selectivity for CH4 toward reacting with NO at high temperatures (T greater than or equal to 450 degrees C) is higher on Co-CLI than on Co-ZSM-5 or Co-FER. On the basic of the overall catalytic results, the importance of zeolite pore size as a structural parameter influencing the activity of intrazeolitic Co2+ ions for NO reduction by CH4 is discussed.