Faraday rotation measurements were performed on TFR 600 using a modified channel of the HCN laser interferometer. The modification was designed to permit simultaneous detection of the integral ʃN e · B ∥ · ds in addition to the interferometric measurement of the line density ʃN e · ds. The purpose of this work was to test the feasibility of an additional polarimetric diagnostic system, without reducing the sensitivity of the present interferometric system, and to check the validity of the polarimetric data obtained under Tokamak conditions. Concerning the interferometer performance, no detriment to the experimental accuracy was observed. By contrast, the polarimetric measurements were affected by various experimental limitations which made quantitative analysis difficult. In the first place, the HCN laser wavelength (337 μm) was too long for probing a typical TFR 600 plasma (max. density = 10 14 cm −3, toroidal field ≳ 4 T), thus introducing problems of beam refraction and elliptical polarization. Second, the optical arrangement was based on the interferometric system which, however, had not been constructed with polarimetric measurements in mind. For this reason certain restraints as, e.g. birefringent Tokamak windows were inevitable. Nevertheless, by comparing the experimental results with a simple theoretical model, good qualitative agreement was achieved, which indicates the feasibility of precise Faraday rotation measurements with a properly constructed device.