Effects of Mo addition on the performance of precipitated Fe catalysts for Fischer-Tropsch synthesis (FTS) were studied in a slurry reactor at 280 degrees C, 1.5 MPa, 2000 h(-1), and syngas H-2/CO= 1.2. The catalysts were characterized by N-2 adsorption, H-2 or CO temperature programmed reduction (TPR), NH3 temperature programmed desorption (TPD), X-ray diffraction (XRD), and Mossbauer effect spectroscopy (MES). It was found that there is a Strong interaction between Fe and Mo, which inhibited the reduction and carburization of Fe catalyst. The surface acidity of the catalysts was also found to increase with addition of molybdenum. In the FTS process, Mo addition decreased the FTS activity of Fe catalyst, markedly enhanced C-12(+) hydrocarbon selectivity, while suppressed C-2-C-8 hydrocarbon selectivity as validated by non-ASF product distribution curves. (C) 2009 Elsevier B.V. All rights reserved.