Interdiffusion of Al and Ga in Al0.4Ga0.6As/GaAs and Al0.3Ga0.2In0.5P/Ga0.6In0.4P quantum wells has been investigated by measuring the photoluminescence of samples annealed at temperatures from 850 to 1065 °C with and without an SiO2cap. At 1000 °C under an SiO2cap, the Al–Ga interdiffusion coefficient is found to be at least two orders of magnitude larger for an AlGaAs/GaAs quantum well compared to an AlGaInP/GaInP quantum well within the same sample. By comparing calculated photoluminescence shifts with measured values, an activation energy of 4.5 eV is found for the Al–Ga interdiffusion in an AlGaAs/GaAs quantum well under an SiO2cap. ©1995 American Institute of Physics.