Zr has been proposed in the patent literature as an important promoter for Co/SiO2 Fischer-Tropsch synthesis (FTS) catalysts. This paper reports on an investigation of the influence of Zr promotion of 20 wt% Co/SiO2 on FTS using catalysts prepared in different ways and having different loadings of Zr (up to 8.5 wt%). The catalysts were investigated using FTS (H-2/CO = 2), H-2-D-2 exchange, and CO dissociation to provide insight into how Zr modifies the Co properties. The Zr-promoted catalysts exhibited higher overall rates of FTS compared to unpromoted Co/SiO2. The sequentially impregnated Co/Zr/SiO2 catalysts appeared to be the most active. However, the coimpregnation method of preparation appeared to result in higher cobalt dispersion. While Zr promotion did not appear to promote or inhibit H-2 activation, hydrogen spillover may have been partly responsible for enhancing the activity of the sequentially impregnated Zr/Co/SiO2 catalysts. Zr also possibly created an active interface with Co that increased catalyst activity by facilitating CO dissociation. Although high levels of promotion tended to increase the selectivity for higher hydrocarbons, Zr appears to be primarily an excellent rate promoter for Co/SiO2.