The iridescent colours produced at the surface of anodically oxidized tantalum have been studied spectrophotometrically. These colours result from interference, the intensity of the reflected light varying between certain maximum and minimum values according to the wave-length. From measurements of the wave-lengths at which the minima occur, from 2600 to 8500 angstrom, the refractive index and its variation with wave-length are deduced. Values are 2$\cdot $84 at 3000 angstrom and 2$\cdot $42 at 8000 angstrom. The phase change at the metallic surface was also deduced, values being 0$\cdot $2$\pi $ radian at 2700 angstrom and 0$\cdot $8$\pi $ radian at 8000 angstrom. The thickness of the anodically-formed films has been found to be 16$\cdot $0 angstrom /V.