Kerr magneto-optical measurements in the transverse orientation were made on ferromagnetic EuO films. These films were cooled to approx 10°K and measured immediately after deposition without exposing the films to air. For incident light polarized in the plane of incidence, the change in intensity of reflected light upon reversal of the magnetization, δ = ΔI/I 0(I0 = reflected light intensity for a demagnetized sample), is large; at a wavelength of 800 mμ, δ = 30%. This may be compared with iron which exhibits a maximum δ ≈ 1% for wavelengths in the visible. Incident light polarized perpendicular to the plane of incidence shows no effect (δ = 0±1%). The transverse Kerr effect exhibits a strong dependence upon wavelength: δ is small at 400 mμ, increases to a maximum at 500 mμ, changes sign at 600 mμ, increases to a maximum at 800 mμ, and then gradually diminishes in magnitude at longer wavelengths. By measuring the phase change as well as the intensity change in the transverse Kerr effect, it is possible to derive experimentally the real and imaginary components of the off-diagonal element of the complex dielectric tensor. Measurements of the longitudinal Kerr effect and Faraday effect in EuO films are discussed in terms of these tensor components. © 1968 The American Institute of Physics.