An experimental and theoretical study of the impact of NO, NO2 and H2O on CO oxidation has been carried out. The experiments were performed in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 800-1400 K. Inlet concentrations of NO and NO2 were 0-1% (vol) and 0-622 ppmv, respectively, while the water vapor level was varied in the ranges 1-32% (NO absent) and 1-10% (NO present). The results show that the concentration of water vapor has a strong effect on the CO oxidation process, partly because it controls the O/OH ratio of the radical pool, and partly due to the high efficiency of H2O in promoting H + O-2 recombination, which causes a strong inhibition of CO oxidation at high levels. Presence of NO and NO2 has a significant impact on moist CO oxidation. In low concentrations NO enhances CO consumption in the 900-1100 K range by converting HO2 to OH. In higher concentrations NO may catalyze recombination of radicals, thereby inhibiting the CO oxidation. The overall effect of NO depends on the radical pool composition, particularly the HO2 level and the O/OH ratio. NO2 is much more efficient than NO in removing radicals and presence of NO, has a strong inhibiting effect on CO oxidation. A chemical kinetic model has been established, which provides a good description of the effect of NO, NO2 and H2O on CO oxidation. Based on the calculations, the rate determining steps in the system have been identified. The present results are significant for understanding the impact of NOx on fuel oxidation and CO burnout in practical systems. Furthermore, they indicate that presence of NO2 impurities may affect significantly experiments on NO containing systems.