Experimental results on the influence of gas phase potassium on CO oxidation under reducing conditions have been obtained. The experiments were performed using a laboratory flow reactor at atmospheric pressure and temperatures in the range 773-1373 K, varying the CO inle concentration and the KCl level. The addition of KCl results in a strong inhibition of the CO oxidation. The inhibition increases with the KCl level. but the effect levels off at high concentrations. The experimental data were interpreted in terms of a detailed chemical kinetic model. In general F satisfactory agreement between experimental data and kinetic modeling was achieved. Analysis of the modeling results indicates that the reaction K + OH + M reversible arrow KOH + M is rate controlling for the radical recombination. The experimental data support a high rate constant for this reaction, but an estimation of the value from the present work is difficult due to uncertainties in the potassium chemistry and in the experimental conditions. Experimental data in a wider range of conditions and alkali precursors are required for model improvement. (c) 2005 Elsevier Ltd. All fights reserved.