The influence of doping of Co/SiO2 catalysts with alumina on a performance of Fischer-Tropsch synthesis (FTS) was studied by extended FTS trials in a fixed-bed tubular pilot-scaled reactor. The addition of small amount of Al2O3 causes apparent promotional effect on the catalysts activity and C5+ hydrocarbons selectivity. The largest promotion effect was observed for the catalysts with 1 wt.% of alumina loading. The modification of the catalyst with alumina (1 wt.%) changes molecular weight distribution of the resultant C5+ paraffins with increasing the fraction of C-8-C-25 and decreasing the fraction of longer chain hydrocarbons. The addition of a proper amount of alumina into Co/SiO2 catalyst alters Co degrees particle size distribution making it narrower with the maximum at 8 nm and the same mean value for Co degrees particle size. A volcano-like dependence of CO chemisorption on alumina loadings with a maximum at 1 wt.% was observed. Relatively high CO chemisorption at the proper amount of alumina decreases the ratio of surface hydrogen to carbon monoxide and in such a way promotes formation of C5+ hydrocarbons. (C) 2016 Elsevier B.V. All rights reserved.