In order to determine the double layer capacitance of the Pt(111) electrode, impedance and capacitance measurements were carried out in neutral and acidic aqueous perchlorate solutions. Separation of the double layer and adsorption contributions of the interfacial capacitance were based on the adsorption impedance theory. The double layer capacitance versus potential plot exhibits a peak at about 0.12 V versus SCE in the 1.5-7 pH range; from here towards cathodic potentials the capacitance attains a value of about 20 muF/cm(2). The peak may be related to the potential of zero free charge of the Pt(111) electrode.