The effect of potassium on nickel/alumina catalysts in relation to CO/H-2 reaction was studied. The examinations were performed on the samples containing 90 wt.% of nickel and up to 6.0 mol% of potassium. XRD studies and adsorption measurements (H-2, O-2 and CO) showed a small effect of potassium on nickel dispersion; at the same time potassium enhanced heat of CO adsorption, which implies that the promoter locates on Ni surface. Temperature-programmed (TP) studies showed that potassium significantly retards the hydrogenation of pre-adsorbed CO and these results were confirmed by steady state tests of CO/H-2 reaction. TP desorption of pre-adsorbed CO provided ambiguous view on the effect of potassium on CO dissociation. Instead, it was revealed that potassium strongly retards hydrogenation of carbon species adsorbed on nickel, which shed a new light on the effect of potassium on CO/H-2 reaction. (C) 2011 Elsevier B.V. All rights reserved.