The structural degeneration of AlAs/GaAs:Be heterostructures epitaxially grown under low temperature conditions and thermally treated is analysed by quantitative high-resolution transmission electron microscopy. Numerical data on the local layer stoichiometry in the proximity of the heterointerfaces is obtained by the analysis of micrographs taken under chemically sensitive imaging conditions. Measured composition profiles are used to calculate diffusion parameters controlling the exchange of aluminium and gallium atoms. Systematic analyses demonstrate that beryllium doping of the GaAs layers decreases the diffusion coefficient for intermixing with undoped AlAs layers by up to one order of magnitude and results in an activation enthalpy enhanced by approximately 0.4 eV compared to undoped quantum wells. The results lead to the conjecture that the diffusion process is governed by the defect related electronic properties close to the interfaces or the formation of defect complexes rather than by the pure concentration of gallium vacancies. (C) 2003 Elsevier Science B.V. All rights reserved.