We have developed a compensation method for measuring the reflectivity of flat highly reflecting dielectric mirror coatings with the help of a laser beam. Using two coupled cavities it is possible to determine the reflectance of a spot with the diameter of the laser beam with an accuracy of about ± 3 × 10−5. The method also allows the measurement of the relative distribution of reflectance over the surface of a single mirror. Here the accuracy is equal to the sensitivity of the method, which is ± 2 × 10−5.