The electrical behaviour of CdTe crystals is controlled by background impurity concentrations and stoichiometric deviations. The stoichiometry control is particularly important in vapour growth. in that even extremely reduced deviations in the starting polycrystalline charge give rise to large increases in excess-component partial pressure. thus limiting the crystal growth rate. A novel procedure was developed for preparing polycrystalline CdTe with highly reduced off-stoichiometry. The method is based on a heat treatment of the CdTe charge and on the control of the MIX Of the Cd and Te-2 vapours through an effusion hole. In situ monitoring of the vapour pressures. and thus of the stoichiometry of the charge was demonstrated. It was shown that the monitoring of the Te-2-partial pressure with time allows to interrupt the process when the desired stoichiometric ratio (Cd/Te approximate to 1 or not equal 1) has been reached. Nominally undoped single crystals could be grown from vapour and melt. with a resistivity of 10(8)-10(9) Ohm x cm, when the Cd/Te atomic ratio was close to unity. A correlation between resistivity and Te-2-partial pressure is reported and discussed. (C) 2002 Elsevier Science B.V. All rights reserved.