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Hydrous ruthenium oxide (RuO 2 •nH 2 O) has inherent proton−electron mixed-conductive nature and offers huge pseudocapacitance (>700 F g −1 ), having attracted the attention of many capacitor engineers. However, the origin of the anomalous pseudocapacitance, exhibiting a strong maximum at a specific narrow optimum annealing temperature of ca. 150 °C, has yet to be understood. Here we show a longawaited explanation for this mystery based on its hierarchical nanostructure unveiled by small-angle X-ray scattering (SAXS). The striking contrast in X-ray atomic scattering factors enables SAXS to exclusively probe heavy RuO 2 in subnanoto nanoscale, dispersed in confined water. We demonstrate that the surface area of the first aggregate of subnano primary RuO 2 particles dominates the accessible number of proton and hence pseudocapacitance, providing critical insights into the nanoarchitectural design of high-performance electrodes for electrochemical capacitors.