The selective catalytic reduction of nitric oxide by methane over copper ion-exchanged natural zeolites was investigated in a packed-bed tubular reactor. The catalytic activity of Cu-N and Cu-H-N catalysts was confirmed as NO, CH4, and O-2 displayed very little reaction in the absence of any catalyst and zeolite without ion exchange was totally inactive. A maximum NO conversion of 33% at 650 degrees C for Cu-N-66 was achieved with 2% NO and 1% CH4 and a contact time of 0.9 g s cm(-3), but the introduction of 2% O-2 reduced the NO conversion to only 12%. Ion exchange for the H-form prior to copper ion exchange was essential for oxygen to promote catalytic activity over the temperature range 250-650 degrees C, with a maximum conversion of 30% at 450 degrees C with O-2 present. The direct reduction of NO by methane was ruled out as a possible reaction pathway.