The thermal stability of two AlGaAs/GaAs single quantum well structures was investigated using room-temperature photoreflectance. Rapid thermal annealing between 800 and 1000 degrees C for times up to 180 s showed spectral "blue" shifts for both the ground state and higher order interband transitions of the quantum well due to Al and Ga interdiffusion across the well/barrier interface resulting in a modificatian of the quantum well confinement profile. The sensitivity of these transition energies to Al-Ga interdiffusion depended on both the quantum well structure and annealing conditions. The results were correlated with a theoretical model, which was developed previously to determine the effects of interdiffusion on the subband structure of quantum well samples, to give characteristics interdiffusion lengths of less than 20 Angstrom for these annealing conditions. These interdiffusion lengths corresponded to limited interdiffusion and was confirmed from polarization sensitivity measurements where the anisotropic polarization behavior of the quantum well was maintained for these annealing conditions. The analysis of these results agrees well with the model and. gives Al/Ga interdiffusion coefficients of similar to 2.9 X 10(-17) and similar to 9.2 X 10(-17)cm(2)/s at 900 and 1000 degrees C, respectively.