The relation between hydrocarbon selectivity and cobalt particle size was studied using different alumina supports. Alumina phases. i.e. delta-, theta- and alpha-Al2O3 were produced from a small and a medium pore gamma-Al2O3 by heat treatment. All catalysts were prepared by incipient wetness impregnation resulting in a cobalt loading of 12, 20 and 30 wt.% cobalt. The catalysts were tested in a fixed bed reactor at 20 bar, 483K and H-2/CO = 2.1. The relation between methane, olefin and C5+ selectivities was studied for 2-14nm cobalt particles supported on delta- and theta-Al2O3 supports prepared from the two different gamma-Al2O3. The C5+ selectivity increases with increasing Co particle size for particles less than 8-9 nm where the selectivity goes through a maximum. The C5+ selectivity decreases for larger Co particles and then seems to approach a constant value independent of the alumina phase and pore size. The lowest CH4 selectivity and the highest olefin to paraffin ratio were obtained at maximum C5+ selectivity. (C) 2012 Elsevier B.V. All rights reserved.