The effect of potassium in the catalytic activity and structural properties of a Ni/Al2O3 catalyst used for the CO2 reforming of methane has been analyzed. To determine the catalytic activity, two types of catalytic test were carried out: (i) temperature programmed reaction (TPR) up to 1173 K and (ii) isothermal reaction (IR) at 973 K. Several techniques were used to characterize the catalysts before and after the reforming reaction: SEM, temperature programmed oxidation (TPO), temperature programmed reduction (TPR-H-2), XPS and XAFS. The presence of potassium facilitates the reduction of nickel during the activation treatment (heat treatment in H-2 at 773 K, 2 h) previous to the reforming reaction, although a further activation of the potassium free catalyst takes place at the reaction condition. Ni/Al2O3 and NiK/Al2O3 catalysts show very similar CH4 and CO2 conversion at 973 K (under TPR and isothermal conditions) and the XAFS analysis of the used samples reveals a similar nickel structure. However, the whole process is different because in Ni/Al2O3 catalysts the formation of coke is much higher. (C) 2003 Elsevier B.V. All rights reserved.