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The stability of structures consisting of p-type high-resistivity epitaxial layers of CdTe grown by metal - organic chemical vapour deposition technology was studied by monitoring the capacitance and the AC and DC conductances of structures. The structures were grown on substrates differing in their conductivity type and composition. The structure parameters were measured in the frequency range 0.5 - 1000 kHz immediately after fabrication, after storage for half a year in the dark at room temperature and after ultrasonic processing. It was found that the stability of the epilayer parameters against ageing and acoustic treatments depended strongly on the substrate characteristics. The ageing and the epilayer degradation due to the ultrasonic processing were found to be a result of the diffusion of mobile acceptor defects from the substrate to the epilayer. The highest electric parameter stability was found for the epi-CdTe/n-CdTe structure. It is shown that the main reason for this stability is the potential barrier at the p - n junction, which prevents the acceptor defects from diffusing out of the substrate.