Pseudocapacitance reaction of hydrous ruthenium oxide was investigated by cyclic voltammetry combined with electrochemical quartz-crystal nanobalance (EQCN) in sulfuric acid as well as in neutral solutions of Na2SO4 and K2SO4. The ruthenium oxide electrode was prepared by attaching the ruthenium oxide particles on gold covered quartz electrode. The results show that there are different types of charge taking place simultaneously during the redox reaction of ruthenium oxide electrode. Their contribution to the overall charge depends on the experimental conditions. Depending on the potential and electrolyte used the redox reaction of ruthenium oxide is accompanied either by mass loss or by mass gain. The average molar masses of the species exchanged between the solid phase and the electrolyte solution depend on the potential and scan rate. The effect of Nafion (TM) top layer was also investigated. It has been found that it does not affect significantly the overall specific capacitance of ruthenium oxide electrode but the apparent molar masses of exchanged species decrease in comparison with the uncovered electrodes. (C) 2010 Elsevier Ltd. All rights reserved.