This study involves the characterization of a series Co/x% Nb2O5/Al2O3 catalysts. The temperature-programmed surface reaction technique (TPSR) and in situ diffuse reflectance spectroscopy (DRS) results have shown a transient behavior of the catalysts, distinguishing very well the Co/Al2O3 from the Co/(Nb2O5Al2O3)-Al-. catalysts. DRS and temperature-programmed reduction (TPR) results indicate the presence of Co2+, besides Co-0 and Co-0-NbOx, which affect greatly the performance of the catalysts. These results allowed us to propose that the interface Co2+-CO0 is responsible for the mathanation reaction, while the Co-0-NbOx is responsible for the hydrocarbon chain grow. The relative amount of each specie on the surface plays a fundamental role on the selectivity behavior of CO hydrogenation. © 2004 Elsevier B.V. All rights reserved.