Introduction of alkaline earth metal oxide (AEMO) into TiO2 was found to promote CO oxidation (in the order BaO>SrO>CaO>MgO) over Au nanoparticles supported on TiO2. From experimental (electron-transfer reaction, CO electrochemical oxidation, CO adsorption and Mott-Schottky plots) and theoretical (density functional theory) calculations, it is proposed that AEMO may act as an electron promoter for CO oxidation, but not as an exclusive structural promoter as widely regarded. Introduction of AEMO raises the Fermi level of the TiO2 support, resulting in an enhanced electron transfer from the support to the Au sites and promoting activation of CO adsorbed at Au sites. Moreover, the enhanced catalytic activities induced by AEMO depend on the increase in the Fermi level of the support in the order: TiO2-BaO>TiO2-SrO>TiO2-CaO>TiO2-MgO. This investigation provides a new insight to understand the role of the electron promoters in the thermocatalytic reactions. (C) 2015 Elsevier B.V. All rights reserved.