We have employed photoluminescence spectroscopy to determine the temperature dependence of the interdiffusion coefficient of Al and Ga in GaAs/Al0.3Ga0.7As quantum wells. The position of the photoluminescence peaks, due to the n=1 electron to heavy-hole transition, was measured before and after annealing the samples. A variational calculation was employed to determine the expected position of these photoluminescence peaks and from this a value of the interdiffusion coefficient was extracted. The interdiffusion process is characterized by an activation energy of about 6 eV leading to an interdiffusion coefficient at 850°C of 4×10 -19 cm2/s. This technique allows for the measurement of small diffusion coefficients in a wide variety of material systems.