Effects of Mo loading on the properties of activated-carbon (AC) supported Fe-Cu-K catalysts and their performance for Fischer-Tropsch synthesis are studied. Physico-chemical properties studied include particle size, reducibility, and dispersion, and catalytic properties include activity, selectivity and stability. Catalysts were characterized by N2 adsorption, energy-dispersive spectroscopy, x-ray diffraction, H2 temperature-programmed reduction and CO chemisorption. Addition of 6% Mo improves catalyst stability without sacrificing activity, but activity is suppressed dramatically over 12% 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 improved stability and higher activity of Mo-promoted catalysts.