Intermixing in InGaAs/GaAs and InGaAs/AlGaAs quantum well structures after rapid thermal annealing with and without an anodic oxide cap on the surface was studied by low temperature (8 K) photoluminescence (PL). The PL peak energy was shifted towards higher photon energies (blueshift) in both types of samples, especially at annealing temperatures above 880 °C. The anodic oxide cap has been demonstrated to inhibit the band-gap blueshift of the quantum well structures. Secondary ion mass spectroscopy data indicated that Ga vacancies were injected from the anodic oxide cap into the epitaxial layers. These vacancies enhanced interdiffusion between group III atoms, and partially relaxed the strain in the structure, resulting in the effect of the suppression of the blueshift.