Transient enhanced intermixing, induced by a focused laser beam, has been observed in arsenic-rich nonstoichiometric AlAs/GaAs quantum wells grown at a low substrate temperature around 300 °C. The intermixing is attributed to a supersaturated concentration of group-III vacancies incorporated into the crystal lattice by the low temperature growth conditions. The intermixing is enhanced by several orders of magnitude relative to diffusion in stoichiometric structures grown at ordinary substrate temperatures. Experimentally observed excitonic blueshifts in laser annealed quantum wells, and the diffusion lengths that are derived from them, are in agreement with those obtained by conventional rapid thermal annealing. Laser annealing provides the capability of spatially patterning integrated optical devices.