The reduction of NOx by hydrogen under lean burn conditions over Pt/Al2O3 is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NOx conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al2O3. In this case, although pure H-2 and pure CO are ineffective for NOx reduction under lean burn conditions, H-2/CO mixtures are very effective. With a realistic (1:3) H-2:CO ratio, typical of actual exhaust gas, Pd/Al2O3 is significantly more active than Pt/Al2O3, delivering 45% NOx conversion at 160 C, compared to >15% for Pt/Al2O3 under identical conditions. The nature of the support is also critically important, with Pd/Al2O3 being much more active than Pd/SiO2. Possible mechanisms for the improved performance of Pd/Al2O3 in the presence of H-2 + CO are discussed.