A magneto-optic Kerr effect (MOKE)-based spectroscopic technique is proposed in the terahertz regime. This method relies on very large reflection edge splitting effects, which occur when the cyclotron frequency (CF) is of the same order of magnitude or greater than the plasma frequency. In the event of a very large reflection edge split, the Kerr rotation (KR) no longer occurs at the plasma edge, but instead occurs at Re(ε+ε−) ≈ 1, within the macroscopic framework of the Drude model. This implies that one can control the spectral occurrence of the MOKE resonance, simply by tuning the magnetic field strength. This phenomenon is unheard of for much shorter wavelengths, due to practical limitations on required magnetic field strengths and hence, can only be realized in the THz regime. A 3 T magnet can easily cover the proposed 0.5–5 THz spectral range for an InSb substrate. Our calculations show that in order to achieve good spectral resolution, the InSb substrate needs to be cooled to 77 K. The Kerr rotation obtained at 77 K for a CF of 1 THz is about 6.28°, which can be increased to about 18.35° by coating a thin low refractive index material on the InSb substrate. A well established ellipsometric technique, which uses four incoherent detectors, is proposed, for fast-simultaneous measurement of KR, Kerr ellipticity and reflectivity.