The diffusion of thin, highlyp‐doped layers in AlGaAs/GaAs single‐ and double‐heterostructures, grown by metalorganic vapor‐phase epitaxy, was studied withC‐Vetch profiling and secondary ion mass spectroscopy. The effect of different post‐growth heat treatments was investigated and diffusion coefficients for both magnesium and zinc were measured. It was found that Mg diffuses about twice as fast Zn and that the order of magnitude of the diffusion coefficient is 10−14cm2 s−1at 900 °C, the exact value being process and concentration dependent. A model based on the interstitial–substitutional diffusion mechanism with suitable kinetic limitations was successfully used to simulate the observed dopant concentration profiles.We also found an anomalous strong diffusion of zinc from GaAs into highlyn‐doped AlGaAs. Detailed results on this and other structures are presented and implications for optimal design of heterostructure devices such as bipolar transistors are discussed.