The effects of two intrinsic deep levels on electrical compensation in semi-insulating CdTe and Cd-Zn-Te crystals are reported here. These levels were found in samples grown by conventional Bridgman and highpressure Bridgman techniques. The levels were observed with thermoelectric effect spectroscopy at distinct temperatures corresponding to thermal ionization energies of Ed15Ev10.73560.005 eV and Ed25Ev 10.74360.005 eV. The first level is associated with the doubly ionized Cd vacancy acceptor and the second level was tentatively identified as the Te antisite (TeCd), which is thought to be complexed with a vacancy. The second level was found to electrically compensate CdTe and Cd-Zn-Te to produce high resistivity crystals, provided that the Cd vacancy concentration is sufficiently reduced during crystal growth or by post-growth thermal processing. The relatively wide band gap, large atomic numbers, and good carrier mobility render CdTe and Cd-Zn-Te ~Refs. 1 and 2! as promising materials for room-temperature radiation sensor applications. High resistivity is necessary but not sufficient for production of CdTe and Cd-Zn-Te radiation detectors. To realize the maximum resistivity of CdTe and CdZn-Te crystals allowed by the band gap a net carrier concentration of 10 8 cm 23 or lower has to be achieved. Early