A new polarimeter is presented which gives time-resolved measurements of both the optic-axis angle and the linear phase retardation for modulated birefringent optical devices. It is suitable for characterizing dynamic waveplate devices based on liquid crystal and other materials. It is fully automated and requires no angular alignment of the device under test. The system has an absolute angle error of < ± 0.3° and a retardance error of < ± 0.44°, with considerably better relative accuracy. The method has been tested with a chiral nematic liquid crystal device exhibiting flexoelectro-optic switching at 3 kHz in the uniform lying helix mode. These results represent the first time-resolved tilt-angle and phase retardation measurements for a liquid crystal device operating at fast switching frequencies.