We study the electrode polarization behavior of different Na-Ca-phosphosilicate glasses by measuring the differential capacitance between blocking Pt electrodes. At low applied dc bias voltages, we detect a linear capacitance regime with interfacial capacitance values considerably larger than expected from mean-field double layer theories and also considerably larger than found for ionic liquids with similar ion concentrations. With increasing bias voltages, the differential interfacial capacitance exhibits a maximum around I V and a strong drop at higher voltages. We suggest that these features are caused by pseudocapacitive processes, namely by the adsorption of mobile Na+ ions at the electrodes. While pseudocapacitive processes are well known in liquid electrochemistry, more detailed studies on solid electrolytes should offer perspectives for improved energy storage in solid-state supercapacitors. (C) 2010 Elsevier B.V. All rights reserved.