Using infrared absorption spectroscopy, we investigate the thermal diffusion behavior of Al atoms from monatomic Al layers embedded in a GaAs epitaxial film. After thermal annealing, the absorption peak of the two‐dimensionally localized vibrational modes at 358 cm−1due to Al layers decreases, while the peak at 362 cm−1due to isolated Al atoms increases. The 362 cm−1peak height is compared with the fraction of isolated Al atoms calculated, assuming the second nearest neighbor hopping diffusion from a monatomic Al layer into GaAs matrix. We thus determine the diffusion coefficient of Al atoms in GaAs to be 2×10−19cm2/s at 700 °C. The present study offers a simple and reliable method to investigate the impurity diffusion in crystals. ©1995 American Institute of Physics.