Anodic oxide films on niobium were studied in situ during formation using a computer-controlled ellipsometer at a wavelength of 6328 A. They were found to be optically homogeneous, isotropic, and nonabsorbing with no field applied but to become anisotropic (optically uniaxial with optic axis normal to the film surface) with field applied. The ordinary and extraordinary indices decreased, and the thickness increased quadratically with applied field. Values are reported for the refractive index of niobium and of the oxide, and these are compared with previous reports. The quadratic electrooptic coefficients for the oxide are reported and are of the same order of magnitude as those for oxygen-octahedra ferroelectrics and somewhat greater than the values for tantalum pentoxide films.