Abstract: In a process or device for very accurate determination of the plane of polarization of polarized light, the light from a light source (1) is polarized by means of a polarizing filter (2) which has a certain setting angle &thgr;0 with respect to the first reference plane, the plane of incidence on the reflecting surface (4). The polarized beam (12) passes through the sample in the measurement chamber (3), in which the angle of rotation is changed by the small angle &thgr;MG. The sum of &thgr;0 and &thgr;MG gives the angle of rotation &thgr;e , at which the beam (13) emerging from the measurement chamber is partially reflected at the surface of a medium of higher refractive index (4).