We present reflection THz-time domain spectroscopy measurements of the complex conductivity ofn-type, 0.038 &OHgr; cm GaAs andn-type, 0.22 &OHgr; cm Si wafers. These measurements clearly demonstrate the efficacy of the reflection technique on highly conductive, optically dense samples and approach the precision of THz–TDS transmission measurements. Because the THz-bandwidth, reflection measurements extend beyond the carrier collision frequency, we obtain direct measures of the mobility and the carrier number density. ©1998 American Institute of Physics.