Fe-Mo interactions and their effect on Fischer-Tropsch synthesis (FTS) performance were investigated over Fe-Mo catalysts. The interaction between Fe and Mo is in the form of Fe-O-Mo structure in as-prepared catalysts and remains largely undestroyed after activation. This interaction leads to increased charge transfer between Fe and Mo species and electron-deficient Fe species with increasing Mo content. The electron-deficient Fe species is responsible for the poor reduction and high methane selectivity in FTS reactions. Meanwhile, Mo incorporation increases the dispersion of iron oxides. The Mo dispersion effect inhibits the aggregation of active iron particles in the heat-treatment. Under the combined action of the electron transfer effect and dispersion effect, the FTS activity of catalysts exhibits a parabola-like trend as a function of Mo content and passes through a minimum at Mo/Fe ratio of 10/100. In contrast, the methane selectivity demonstrates an opposite tendency to FTS activity. (C) 2010 Elsevier B.V. All rights reserved.