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We report the results of longitudinal magneto‐optical Kerr effect (LMKE) measurements on bilayers consisting of a thick permalloy film covered by varying thicknesses of Nb. The LMKE pertains to situations in which light with a finite angle of incidence is reflected from a plane surface, and M in the sample is parallel both to the plane of incidence and the surface. Either s‐ or p‐linearly polarized 6328‐Å light is incident on the sample at 60° to the normal. The two components of the complex Kerr rotation are measured with a modulation technique utilizing a λ/4 plate and a Faraday rotator driven by dc and 2000‐Hz signals. With cycling of an in‐plane magnetic field, square magneto‐optical hysteresis loops are observed in both rotation and ellipticity. There is a change in sign, as well as a decrease in the magnitude of the switching discontinuity in these quantities with increasing overlayer thickness. These results are very similar to those of the corresponding idealized boundary value problem. The technique gives experimental information on the magnetic properties of films buried below a nonmagnetic material or of thin films bounded on both sides by a nonmagnetic material. Further, it gives a probe of the phase and intensity of optical frequency electric fields below a reflecting surface.