SO2 depolarized electrolysis (SDE) is an important reaction in the hybrid sulfur process, which is one of the most promising approaches for mass hydrogen production without CO2 emission. The potential loss of SDE process could be separated into four components, i.e., reversible cell potential, anode over potential, cathode over potential, and ohmic loss. Thus far, the total cell potential for the SO2 depolarized electrolyzer has been identified to be dominantly controlled by the sulfuric acid concentration of the anolyte and the temperature of the electrolysis process. In this work, an in-situ electrochemical impedance spectroscopy measurement of the anodic SDE reaction was conducted. The results show that anodic over potential is mainly caused by the SO2 oxidation reaction rather than ohmic resistance or mass transfer limitation. This study extends the understanding of the SDE process and gives suggestions for further improvements in SDE performance. (C) 2015 Elsevier B.V. All rights reserved.