It was found that air-exposure suppressed the sequent reduction behavior before a Fe-Mn-Cu-K/SiO2 Fischer-Tropsch synthesis (FTS) catalyst was loaded in reactor. Thermogravimetry and mass spectrometry analysis (TG-MS), in situ diffuse reflectance infrared Fourier transform (DRIFT) analysis, CO2 temperature-programmed desorption (TPD), in situ syngas reduction and Mossbauer spectroscopy were used to reveal the intrinsic relationship between air-exposure and reduction behavior of catalysts. The result of in situ reduction indicates that the air-exposure restrains the reduction of Cu-promoted catalyst and suppresses the formation of active sites. Results of TG-MS indicate that water and CO2 are adsorbed on surface of catalysts when catalysts were exposed to air. Copper promotion enhances the selective adsorption of CO2 in air. The species formed upon CO2 adsorption are irreversible surface carbonates, which cannot be removed under the typical or higher temperature used in FIFS reaction. The surface carbonates formed in air-exposure restrain the role of Cu in reduction and lead to the low extent of reduction or carburization. (c) 2005 Elsevier B.V. All rights reserved.