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Abstract: Nanoporosity in silicon leads to completely new functionalities of this mainstream semiconductor. In recent years, it has been shown that filling the pores with aqueous electrolytes, in addition opens a particularly wide field for modifying and achieving active control of these functionalities, e.g., for electrochemo-mechanical actuation and tunable photonics, or for the design of on-chip supercapacitors. However, a mechanistic understanding of these new features has been hampered by the lack of a detailed characterization of the electrochemical behavior of mesoporous silicon in aqueous electrolytes. Here, the capacitive, potentialcontrolled charging of the electrical double layer in a mesoporous silicon electrode (pore diameter 7 nm) imbibed with perchloric acid solution is studied by electrochemical impedance spectroscopy. Thorough measurements with detailed explanations of the observed phenomena lead to a comprehensive understanding of the capacitive properties of porous silicon. An analysis based on the Mott-Schottky equation enables the determination of essential parameters such as the flatband potential, the carrier concentration and the width of the space charge region. A comparison with bulk silicon shows that the flatband potential in particular is significantly altered by the introduction of nanopores, as it shifts from 1.4 ± 0.1V to 1.9 ± 0.2V.