The influence of small amounts of Pt on supported Co catalysts has been investigated by several methods. Catalysts containing 9 wt.% Co and 0 or 0.4 wt.% Pt on Al2O3 or SiO2 were prepared by coimpregnation. At T = 483 K, P = 1 bar, and H-2/CO = 7, CO hydrogenation rates (based on weight of cobalt) for Pt-promoted catalysts were 3-5 times higher than those on their unpromoted analogues, The selectivity was not influenced by the presence of Pt. TPR-studies have shown that the presence of Pt strongly influences the reducibility of the catalysts, with TPR-peaks shifting to lower temperatures for all catalysts. This effect is most pronounced on the Al2O3-supported catalysts, where the highly dispersed and otherwise difficult to reduce surface cobalt oxides are readily reduced at normal reduction temperatures (623 K) in the presence of Pt. The dispersion of metallic cobalt on Pt promoted catalysts increased compared to that on the unpromoted catalysts, with Al2O3-supported catalysts showing the largest effect due to the reduction of highly dispersed surface cobalt oxide. Apparent turnover numbers (based on H-2-chemisorption) were higher for Pt-promoted catalysts than for their unpromoted analogues by a factor of similar or equal to 2. Decoupling coverage and intrinsic reactivity effects on the apparent turnover number by the use of steady-state isotopic transient kinetic analysis revealed essentially constant true turnover numbers on all catalysts. Thus, the increased apparent turnover numbers on Pt-promoted catalysts are due to a higher coverage of reactive intermediates.