The Modulation of Interface Capacitance Transfer Function (MICTF) technique in combination with Electrochemical Impedance Spectroscopy (EIS) and derived transfer functions has been developed for the study of double-layer relaxation and adsorption processes. Using a Hg electrode in NaBr and KCl solutions as examples, the MICTF is shown to be well-suited for the investigation of physico-chemical processes occurring inside the diffuse layer. Two time-constants are found and their characteristics are analyzed as a function of the potential of zero charge (PZC), the electrolyte concentration, and the nature of the electrolyte. Interpretation is discussed in the framework of accepted double-layer theories including relaxation in the diffuse double-layer and adsorption processes.