Photoluminescence (PL) measurements were performed in order to investigate the carbon impurity effects on the intermixing behavior of GaAs/AlAs multiple quantum wells (MQWs) grown by molecular‐beam epitaxy. The GaAs/AlAs MQWs were annealed with a carbon source in a furnace annealing system. The PL spectra show that the magnitude of the intermixing of Al and Ga induced by thermal annealing in GaAs/AlAs MQWs increases with depth. This behavior is not in agreement with the intermixing mechanism considering vacancy injection of the surface. The nonuniformity of the intermixing as a function of the depth originated from the carbon impurities which were injected during thermal treatment. ©1995 American Institute of Physics.