The effect of catalyst pretreatment, using hydrogen or carbon monoxide, on the activity and selectivity of cobalt on ZnO catalyst (10 wt% Co/ZnO) during Fischer-Tropsch synthesis was studied in a fixed bed reactor. Catalyst reduced with hydrogen had higher activity and higher olefin content, and produced less methane than the CO activated catalyst. Catalysts were characterized by means of Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR), and hydrogen chemisorption (H(2)-TPD) with pulse re-oxidation. Reduction of Co(3)O(4) occurs in two stages during H(2)-TPR and CO-TPR. In the latter case CoO, metallic cobalt, cobalt carbides (Co(x)C; x = 2 or 3) and carbon (from CO disproportionation) are formed during the reduction process. The existence of Co(0) along with CoO and Co(x)C, indicates a possibility of two reaction paths for formation of CoxC from CoO: the direct path (CoO -> Co(x)C) and a series reaction via Co(0) (i.e. CoO -> Co(0) Co(x)C). (C) 2011 Elsevier B.V. All rights reserved.