The effects of a dielectric overlayer on Kerr magneto-optic properties in thick EuO films were examined. The dielectric film serves not only as a passivation layer, but also to increase the Kerr effects. A series of thin Eu2O3 films were vacuum deposited on a thick EuO film, followed by in situ measurements of the reflectivity and the transverse Kerr effects using a polarized light (with the electric field vector in the plane of incidence) at 45°angle of incidence in the wavelength region from 0.4 to 0.8 μ. As the dielectric (Eu2O3) thickness increases the characteristic reflectance minimum of EuO shifts toward longer wavelengths, accompanied by decreasing reflectivity. This shift agrees well with calculated reflectances using optical constants of Eu2O3 film and EuO. The minimum reflectance of ∼0.04% occurs at 0.675 μ for a dielectric thickness of ∼500 Å. The transverse Kerr effects also depend strongly upon the dielectric thickness. The maximum Kerr effects occur at the wavelength of minimum reflectance For example, corresponding to the minimum reflectance of ∼0.04%, a maximum transverse Kerr effect of ∼+140% occurs at this wavelength. This value can be compared with ∼25% in bulk crystal at 0.575 μ and ∼80% in thin EuO film on a mirror substrate at 0.525 μ, which is the largest value reported in the literature. Preparations and optical properties of several other rare-earth oxide films are also presented. © 1969 The American Institute of Physics.