GaAs/AlGaAs quantum wells (QWs), selectively intermixed by SiO2capping and rapid thermal annealing, have been characterized on a microscale using spatially resolved photoluminescence (PL) spectroscopy. From the evolution of the PL spectra across the boundary between the unmixed and intermixed regions, it was concluded that the transition region is narrower than the 1.5 &mgr;m excitation beam diameter. The magnitude of the intermixing was also found to increase with the thickness of the oxide. The present intermixed QWs were found to be stable against subsequent thermal treatment below the temperature limit imposed by the intrinsic interdiffusion. These results demonstrate that the effective band gap of QWs, and their physical properties, can be adjusted by controlling the oxide thickness profile.