The method for in situ characterization of surface transformations on heterogeneous catalyst is described. The method is based on the measurements of thermal electron and ion emissions from the catalyst surface. It is demonstrated by investigating the depassivation process of undoped and K-doped Mo2C catalysts exposed to air after synthesis and use to identify characteristic steps of reduction-carburization by flowing CH4/H-2 gas mixture. The information obtained from thermal emission experiments is consistent with the methane consumption profile, which provided an independent observation of the recarburization step. (c) 2005 Published by Elsevier B.V.