Fischer-Tropsch synthesis of low molecular weight (C-2-C-4) olefins is a valuable alternative process for the production of key chemicals from non-petroleum precursors such as a renewable C source like biomass. The influence of the H-2 treatment temperature of zirconia-supported iron catalysts on the conversion of synthesis gas has been investigated. The CO conversion rate, based on steady-state, increased with the pretreatment temperature up to a maximum and then decreased at higher temperatures. Moreover, methane selectivity was found to decrease slightly from the lowest H-2 treatment temperatures (46%) to the highest ones (34%), while C-2-C-4 and C5(+) hydrocarbons followed an opposite trend. Presumably, methane formation takes place at highly active low coordination sites residing at corners and edges, which are more abundant on small iron carbide particles. Lower hydrocarbons are mainly produced at terrace sites that are available and active, quite independent of the iron crystallite size. (C) 2015 Published by Elsevier B.V.