The influence of carbon dioxide on nitric oxide reduction by carbon monoxide catalyzed over a porous limestone Stevns Chalk was studied in a fixed bed quartz reactor at 850 degrees C under alternating oxidizing and reducing conditions, and under constant reducing conditions. The presence of carbon dioxide was found to inhibit the catalytic nitric oxide reduction. The nitric oxide reduction by carbon monoxide was found to fit the power law expression -r(NO)=k .[NO](1) .[CO]((0.8-0.9)).[CO2](-0.7) at ambient pressures, for 300-1200 ppmv NO, 2-6 vol.-% CO and 2-16 vol.-% CO2. A simple mechanism for the nitric oxide reduction is proposed and a rate expression which agrees with the experimental results is derived. This study indicates that investigations of nitric oxide reduction by carbon monoxide catalyzed by CaO should be carried out in the presence of relevant concentrations of carbon dioxide.