Spinel-type binary transition metal oxides of cobalt and nickel (or copper) with composition MxCo3-xO4 (where M = Ni, Cu; 0 less than or equal to x less than or equal to 1) were synthesized in film forms by a sol-gel route and their physicochemical and electrocatalytic properties have been investigated using SEM, XRD, cyclic and stationary voltammetries. The roughness factors, R-F, of electrodes made of these films were determined by AC-impedance and cyclic voltammetry (CV). The two procedures compared well, and enabled the discrimination between apparent and true (real) electroactivity towards the oxygen evolution reaction (OER). Intermediate compositions showed a higher activity despite a lower real surface area, and laboratory tests at 70 degrees C in 30 wt% KOH good practical performances. (C) 2000 Elsevier Science Ltd. All rights reserved.