Impurity‐induced layer disordering experiments on AlxGa1−xAs‐GaAs quantum well heterostructures (QWHs) that are doped heavily with carbon are described. The data show that carbon doping retards Al‐Ga interdiffusion relative to an undoped crystal, and that interdiffusion in C‐doped QWHs is not enhanced by a Ga‐rich (versus As‐rich) annealing ambient. The data are inconsistent with most Fermi‐level‐effect models for layer disordering that do not include chemical impurity dependence or sublattice dependence, and that do not consider the possibility of inhibited Al‐Ga interdiffusion in extrinsic crystals.