  We present high precision measurements of polarization rotations in the
frequency range from 0.1 to 2.5 THz using a polarization modulation technique.
A motorized stage rotates a polarizer at ~80 Hz, and the resulting modulation
of the polarization is measured by a lock-in technique. We achieve an accuracy
of 0.05{\deg} (900 {\mu}rad) and a precision of 0.02{\deg} (350 {\mu}rad) for
small rotation angles. A detailed mathematical description of the technique is
presented, showing its ability to fully characterize elliptical polarizations
from 0.1 to 2.5 THz.
