A magneto‐optical (MO) Kerr tracer based on an ellipsometer was developed for studying the surface magnetic properties of thin films. This system can be used to measure the optical properties n and k, which are strongly related to the magneto‐optical parameters θk and ηk. In order to carry out measurements on materials having a small θk we have developed a multiple reflection sample holder which gives an enhancement of the rotation. The behavior of the sample holder is explained in terms of Jones matrices. The optical and magneto‐optical properties for a thickness series of Co‐Cr films were measured. Comparative measurements on other systems gave similar results.