PbTe with a bandgap E = 0.21 eV (77 JS) is an important semiconductor for optoelectronic devices in the medium infrared region. Thallium ions T1' a r e known as acceptors in PbTe, if they a r e substituting lead ions, Pb . The electrical efficiency of T1 in PbTe should be poor /lh nevertheless there has been found a different electrical behaviour after 'additive' and 'substitutional' incorporation, respectively /2/, and even a rather high electrical activity of T1' /3/, a t least a t concentrations cT1 5 0.3 at% /4/. Of course, one has to take into consideration the electrical activity of point defects due to nonstoichiometry, dependent on the kind of producing the crystal, a t least at small dopant concentrations. centration. Fig. 2 shows that p77K% ( to 5 )~T l .