Ab initio atomistic thermodynamics is utilized to achieve the understanding of the surface structure and stability of chi-Fe5C2 under CO and synthesis gas (H-2/CO) pretreatment conditions in catalyst activation for Fischer-Tropsch synthesis (FTS). On the basis of the computed surface free energy (gamma) as a function of the carbon chemical potential (mu(C)) by considering temperature, pressure, and H-2/CO ratios, it is found that CO pretreatment favors stable carbon-rich facets, while small amount of H-2 added into CO leads to a large decrease in mu(C) and thus stabilizing carbon-poor facets. The high activity of surface carbon toward hydrogenation might explain the enhanced initial activity of the FTS catalysts activated from CO pretreatment. Moreover, under both CO and H-2/CO pretreatments, either low temperature or low pressure can lead to stable carbon-rich facets. (C) 2012 Elsevier Inc. All rights reserved.