Effects of carburization during pretreatment with different atmospheres on K promoted Fe-Mn catalysts for CO hydrogenation were investigated. The bulk structure, surface composition and chemical properties of the catalysts before and after pretreatment were characterized by means of X-ray diffraction (XRD), Raman spectroscopy, in situ X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption/oxidation (TPD/TPO) and catalytic activity tests for the selective formation of light olefins. Results indicated that the surface basic sites and CO adsorption were promoted after pretreatment, especially for CO pretreatment. As the formation of iron carbide and elevation of carburization temperature, the surface basicity was significantly enhanced. The catalysts after CO or syngas pretreatment exhibited much higher selectivity to light olefin than that pretreated with H-2 due to the basicity change during the pretreatment. The formation of carbide species improved the surface basicity which enhanced the adsorption and dissociation of CO and then decreased the second hydrogenation of primary olefins. (C) 2013 Elsevier Ltd. All rights reserved.