The real surface areas of Co3O4 films have been measured by Zn2+ adsorption. Co3O4 films have been prepared by spray pyrolysis. The results are compared with crystallite sizes obtained by X-ray diffraction measurements and roughness factors calculated from cyclic voltammograms. Discrepancies between the three methods are discussed and examined. (C) 1997 Elsevier Science Ltd. All rights reserved.