Data on optical reflectance and anodization voltage, obtained during the galvanostatic anodization of metallic niobium foils in an H3PO4(1%) solution at room temperature were simultaneously recorded as a function of time, to determine the thickness of the Nb2O5 films formed. From these data, plots of film thickness vs anodization voltage were obtained. A linear relation was always observed and in all cases but one, an angular coefficient of 22 angstrom V-1 was verified.36812971300