The reflectivity of anodized electropolished tantalum was measured as a function of angle of incidence for 4358Aå light polarized in the plane of incidence. The principal aim of these measurements was to obtain the refractive index of the oxide. This is needed to determine absolute thicknesses of oxide by the spectrophotometric method in which the wavelengths of minimum reflectivity due to interference at near normal incidence are determined. The measurements also provided a method of determining the optical constants of the metal, without assuming that the surface is perfectly flat or that an unanodized surface is free from oxide. It was found that films formed on tantalum in have a thin outer layer of light‐absorbing oxide. The bulk of the film is, however, nonabsorbing and homogeneous. A large part of the thickness of films formed in concentrated sulfuric acid absorbs light. The refractive index at 4358Aå wavelength of films formed in dilute solution increases by about 0.3% per tenfold decrease in the current density at which they are made and by about 0.06% per 10°C rise in the temperature at which they are made. The refractive indices of films formed on niobium, on 25 a/o Ta + 75 a/o Nb, and on 25 a/o Nb + 75 a/o Ta were estimated and a preliminary study was made of films on zirconium.