The current study examines the effect of downstream synthesis gas feeding on Fischer-Tropsch product distributions in two series plug flow reactors over a precipitated iron catalyst. Synthesis gas at a constant H-2: CO ratio of 2.5 was fed into the entrance of a first reactor, and the resulting products were fed into a second reactor along with fresh synthesis gas of varying H-2:CO ratios (3, 2. 1, and 0.5). The selectivity of C5+ hydrocarbons in the downstream feeding cases was over two times lower than the corresponding "top feeding" cases, in which an equivalent amount of synthesis gas was fed through both series reactors without the addition of downstream synthesis gas. These observations were attributed to the effects of bed residence time and H-2:CO ratios as influenced by the simultaneous water-gas shift reaction. (C) 2009 Elsevier B.V. All rights reserved.