In this work Co, Mo and CoMo catalysts supported on a montmorillonite clay (STx-1, Texas, USA) in its natural and Al-pillared forms were prepared. These catalysts in their unreduced and reduced forms were tested over the dehydrogenation of ethylbenzene to study the effect of the metal reduction. The catalysts were prepared by impregnation with aqueous solutions of CoCl2 center dot 6H(2)O and (NH4)(6)Mo7O24 center dot 4H(2)O and characterized by N-2 adsorption, X-ray diffraction, temperature programmed reduction and CO chemisorption. The total metal content of cobalt and/or molybdenum was 4 wt.% in the monometallic and bimetallic catalysts, varying the Co-Mo mass ratio in 1:3, 2:2 and 3:1 (Co:Mo atomic ratios of 0.54, 1.63 and 4.84) in the latter. The surface areas of the natural and Al-pillared clays were 72 and 123 m(2)/g, while the areas of the Co, Mo and CoMo catalysts supported on ST and Al-ST were 48-69 and 77-89 m(2)/g. The reduction profiles revealed the presence of different Co and Mo species and the existence of metal-support interactions. The use of cobalt chloride formed particles of large sizes and Cl- blocks the active sites. The reduced catalysts showed greater activity than the unreduced catalysts at 400 degrees C. The most active catalysts were the one that contained 4 wt.% of Mo (in the monometallic) and that with the higher Mo content (in the bimetallic). (C) 2006 Elsevier B.V. All rights reserved.