An equivalent circuit model for electrochemical double layer capacitors (EDLCs) is proposed through analyzing the electrochemical impedance spectroscopy (EIS) measurements. The model is developed based on the Grahame theory, while these capacitive or resistive behaviors in the presence of charge diffusion and the ion adsorption at the double layer interface and bulk media are investigated. This circuit model, upon its validation against the EIS data, is successfully applied to characterize the practical EDLC devices. Meanwhile, experimental results are obtained from different EDLC cells that consist of the activated carbon-based electrodes and two electrolytes, namely, aqueous (H2SO4) and organic (Et4NBF4/PC). The model predicts the useful parameters (such as resistance and capacitance) which help interpret electrochemical reactions at the electrode/electrolyte interface. The quantitative dependence of impedance on the applied electrode potential is analyzed for two electrolytes during charging/discharging, and its correlation with the internal resistance (referred ESR) is studied.