The interface Hg/HClO4 . 5.5H(2)O was investigated by cyclic voltammetry and electrochemical impedance spectroscopy between 170 and 300 K. In this temperature range, phase transitions occur for electrode and electrolyte, such that within 6 K the electrode-electrolyte interface changes from solid-solid over solid-liquid to liquid-liquid or Vice versa. The double layer capacity was investigated as a function of potential for different temperatures and states of aggregation of the respective phases.