Electrochemical impedance spectroscopy (EIS) is largely used in studying water splitting reaction in bipolar membranes (BPMs). This reaction is also very important in monopolar membrane systems, where it constitutes one of the possible mechanisms of the overlimiting current conductance. However, despite of GO-year research history in this domain, there are only a few studies where EIS was applied. Apparently, it is due to poor understanding of AC response of monopolar membranes in overlimiting current range where charge transfer by electrodiffusion competes with current-induced convection and water splitting products transfer. For the first time, we propose a mathematical description of monopolar membrane's impedance spectra associated with water splitting. We show that these spectra are of the Gerischer type, as in the case of BPMs. The boundary value problem takes into account the proton-transfer catalytic mechanism of water splitting. The solution of this problem is in a good accordance with experimental spectra found for an AMX-SB membrane at differenat biased DC currents. It is found that the effective rate constant of water splitting reaction increases with increasing DC current, similar, as in the case of BPMs. (C) 2015 Elsevier B.V. All rights reserved.