The effects of Mo loading (0-12 wt%) on the properties of activated-carbon-(AC-) supported Fe-Cu-K catalysts and their performance for Fischer-Tropsch synthesis are studied. Physicochemical properties studied include particle size, reducibility, and dispersion, and catalytic properties include activity, selectivity, and stability. Catalysts were characterized by N-2 adsorption, energy-dispersive spectroscopy, X-ray diffraction (XRD), H-2 temperature-programmed reduction (TPR), and CO chemisorption. Catalyst performance was studied at 310-320 degrees C, 2.2 MPa, 3 Nl/g-cat/h, and H-2/CO = 0.9. Reaction results in a fixed-bed reactor show that addition of 6% Mo into the Fe-Cu-K/AC catalyst improves catalyst stability without sacrificing activity, but activity is suppressed dramatically on a 12% Mo-loaded catalyst. Detectable hydrocarbons of C-1 to C-34 are produced on the Fe-Cu-K/AC catalysts with or without Mo. However, the addition of Mo results in the production of more CH4 and less C5+ hydrocarbons. The Mo promoter greatly enhances secondary reactions of olefins, leading to a large amount of internal olefins (i.e., other than 1-olefins) in the product. TPR shows that a strong interaction between Fe and Mo oxides is present, and the extent of reduction of Fe is suppressed after addition of Mo to the Fe-Cu-K catalyst. CO-chemisorption and XRD studies show increased iron dispersion and decreased particle size of the iron carbide and iron oxide after the addition of Mo. Segregation of iron active sites, thereby preventing them from agglomerating, and a larger number of active sites on the 6% Mo catalyst are possible reasons for the improved stability and higher activity of Mo-promoted catalysts.