Transition-metal oxides and noble metals are wellknown selectivity and activity promoters in cobalt-based Fischer− Tropsch catalysis. Niobia has been shown as an effective selectivity promoter as support material; however, its low porosity limits the cobalt loading. To combine the selectivity-promoting properties of niobia with a highly porous support, niobia-modified silica was prepared and applied as a support for Co and PtCo catalysts with cobalt loadings up to 21 wt %. Niobia promotion was found to increase the C 5+ selectivity at 1 bar; however, it appeared to be ineffective at 20 bar. Promotion of Co/SiO 2 by a combination of platinum and niobia yielded an increase of the cobalt-weight normalized activity by a factor of 2−3 in the case of amorphous niobia and by a factor of 3−4 with niobia nanocrystals present, due to both an increased number of active sites and an increased cobalt-surface specific activity (turnover frequency).