Effects of Mo addition on the Fischer-Tropsch synthesis (FTS) performance of precipitated Fe and FeCu catalysts were tested at 280 A degrees C, 1.5 MPa, H-2/CO = 2.0, 2,000 h(-1). Reaction results in fixed-bed reactor indicate that Mo addition stabilizes the selectivity to higher molecular-weight hydrocarbons of Fe and FeCu catalysts. Carbonaceous species over the spent catalysts were studied by Mossbauer effect spectroscopy (MES) and temperature-programmed hydrogenation (TPH). It is found that Mo addition inhibits the carbon deposition on iron catalysts surface during the FTS reaction and thus probably protects the active sites for producing heavy hydrocarbons.