Pseudocapacitive redox reaction of hydrous ruthenium oxide was investigated by the combined electrochemical and quartz crystal nanobalance measurements on gold support in H2SO4 and Na2SO4 solutions. The results show that the pseudocapacitance arises from at least two different Faradaic reactions with significant influence of double layer capacitance. All three processes simultaneously take place during charging/discharging reaction, but their contribution vary depending on the electrolyte used, the temperature pre-treatment and on the potential range. One Faradaic reaction releases protons during oxidation reaction resulting in the electrode mass decrease, while another Faradaic reaction results in the chemical binding of water leading to the mass gain during the oxidation reaction. The former reaction is favoured in acidic electrolyte and at lower anodic potentials, and the latter reaction proceeds predominantly in neutral media and at higher anodic potentials. The influence of annealing temperatures on the characteristics of the redox reaction of hydrous ruthenium oxide, as well as on its capacitance, was confirmed. It was demonstrated that specific capacitances of hydrous ruthenium oxide could achieve values as high as 1500 F g(-1), provided that conditions of good electronic conductivity among RuO2 particles, as well as good electrical contact between gold and RuO2, are met. (C) 2011 Elsevier B.V. All rights reserved.