In order to reduce CO2 emissions there is a trend towards an increased use of more fuel-efficient lean burn combustion engines and thus an increased need for deNO(x) catalysts for such engines. In this study a mechanical mixture of COFER and HZSM-5 zeolites was studied as a dual pore system catalyst for the continuous lean reduction of NOx with isobutane as reducing agent. The degree of NOx conversion was found to be strongly dependent on the oxygen concentration in the gas mixture, and the highest degree of reduction was achieved with 10% O-2. This optimum is explained by the need for NO2 as an intermediate for the catalyst to work efficiently, and that above a certain oxygen concentration O-2 oxidises the hydrocarbon resulting in a lower reduction potential for NOx. At an operating temperature of 350degreesC the conversion of NOx to N-2 was 52% under steady-state conditions, but the efficiency can be improved further by optimisation of the oxidising zeolite and of the mixing ratio of the two zeolites. (C) 2003 Elsevier B.V. All rights reserved.