The small-signal impedance of electrolyte/insulator/silicon structures is partly determined by the properties of the insulator/electrolyte interface. A theoretical model for this interfacial impedance is derived. Two parallel contributions are involved: the double-layer capacitance, for which a Gouy-Chapman-Stern model is adopted, and a branch containing the capacitance related to the surface reactions with H+ and OH− ions from the electrolyte. These surface reactions cause the total interfacial impedance to be very low for insulators with a high surface reactivity such as, for instance, Al2O3 or Ta2O5. For SiO2 surfaces, the reactivity is much lower, implying a larger interfacial impedance. Measurements of the interfacial impedance were carried out at low frequencies on 12 nm SiO2 layers in NaCl electrolytes at ionic strengths of 10−4, 10−3and 10−2 M. The results agreed with the theoretical predictions which were based on parameter values obtained from independent measurements of ψ0/pH characteristics. The agreement confirms the model for the formation of the surface charge through reactions of fixed silanol groups in the SiO2 surface.