The kinetics of formation of insulating oxide films on zirconium was studied working at a constant voltage of 72,9 V. The results obtained were used to test two theoretical expressions for metal-oxide film formation kinetics.A value of 1·24 eV was obtained for the interfacial barrier height using the Mott-Cabrera theory and assuming a fixed half-jump distance of 2·5 Å and a limiting, saturation value for the concentration of active sites for oxide growth at the metal surface.With the Dewald theory, values of 1·58 and 1·73 eV were obtained for the interfacial and internal barrier heights, respectively, and 2·9 and 4·6 Å for the corresponding half-jump distances.Some evidence is presented for variation of the concentration of metal surface active sites with film formation rate.