Fischer-Tropsch (FT) synthesis was performed over 15%Co/CeSi and 15Co/CeMnSi catalysts using a 1 L CSTR at 230 degrees C, 1.89 MPa and a H-2/CO ratio of 1.86. The oxygenate selectivity for both catalysts increased with increasing time-on-stream during FT synthesis. X-ray diffraction, EXAFS and HR-TEM analyses of fresh catalysts and those withdrawn during FT synthesis indicate that increases in the selectivity for oxygenates with time are related to structural changes in the catalyst, most likely associated with a transition from a metallic cobalt surface to one containing surface cobalt carbide. Moreover, a significant effect of carburization on increasing the selectivity for oxygenates was observed for FTS using the 15Co/CeMnSi catalyst. The reactivity of CO was inhibited by the presence of carbon on the surface of cobalt which promoted oxygenate formation through a CO insertion mechanism. (C) 2015 Elsevier B.V. All rights reserved.