To obtain a novel, active and selective to diesel catalytic material for syngas processing via Fischer-Tropsch synthesis (FTS), a series of 20 wt.% cobalt catalysts has been prepared by impregnation of a mesoporous molecular sieve based on silica (SBA-15, Al-MCM-41, INT-MM1), and a commercial amorphous silica forcomparison purposes. All materials were characterized by several physico-chemical techniques: AAS, BET surface area, XRD, TPR, and H-2 chemisorption with pulse reoxidation and finally their reactivity on the FTS reaction was evaluated at 523 K, 10 bar, and (HCO)-C-2/ = 2. Catalytic and characterization results show a great influence of mesoporous support porosity on the structure, reducibility, and FTS catalytic behavior of cobalt oxide species supported over these ordered materials. It was found that the size of supported cobalt oxide species formed during the calcination step increased with the average pore size (D-p) of the mesoporous support. Thus, the catalyst with larger Co oxide species located in wide pore silica showed to be easily reducible, more active and very selective toward the diesel fraction. It seems to be the case of the Co/SBA-15 solid, which showed to be the most active solid (XCO similar to 63%) when the same mass of catalyst was used. Under CO iso-conversion conditions (XCO similar to 40%), Co/SBA-15 was more selective toward the formation of C5+ hydrocarbons (similar to 80%. alpha = 0.76) and less selective to CH4 (similar to 15%). On the contrary, when Al-MCM-41 and INT-MM1 were used as supports, a lower selectivity to C5+ and CO conversion and higher CH4 selectivity (similar to 20%) were observed due to the decrease of D-p, of the cobalt oxide species size and the reducibility degree of such species. (C) 2009 Elsevier B.V. All rights reserved.