A simple experimental approach has been employed to obtain thermochemical parameters for cation vacancy formation and migration in the AlGaAs heterostructure system. Cation vacancies are injected into the free surface by annealing under an arsenic-rich ambient. Their presence is detected by monitoring the local rate of Al–Ga interdiffusion at imbedded quantum well markers. The sample is unusually thick allowing us to separately identify contributions from the vapor, epilayer, and substrate phases. The entropies and enthalpies of vacancy formation and migration are(5.2±5.7) kBand(1.8±0.5) eVand(11.3±4.4) kBand(3.3±0.4) eV,respectively. ©1998 American Institute of Physics.