We report on the first successful generalized Mueller-matrix ellipsometry measurements in the THz-frequency domain using the high-brilliance THz synchrotron radiation source IRIS at the electron storage ring BESSY, Germany. Generalized Ellipsometry, which is known as a powerful tool for measurement of optical constants including anisotropy and which was previously used in the FIR to VUV spectral range, is now employed for the first time to investigate condensed matter samples in the frequency range from 0.9 to 8 THz (30 to 650 cm -1 ). Exemplarily, results obtained from bound and unbound charge-carrier investigations in low-dimensional semi- and superconducting systems are presented. Future applications of this technique for investigation of charge-carrier dynamics in magnetic fields are envisioned.