Co3O4 has been prepared by thermal decomposition of basic CoCO3 in the temperature range 200 to 800 degrees C. The pzc has been measured by potentiometric titration and has been found to coincide with the iep determined by microelectrophoresis. The pzc is constant at ca. 7.3 between 200 and 450 degrees C. The GCSG model of the electrical double layer has been applied to the experimental data to derive the real surface area of the oxide and the inner layer capacitance. The electrochemical surface area coincides with BET data. The results of this work have been compared with those obtained previously with Co3O4 prepared by thermal decomposition of the nitrate.