The mixing of GaAs/GaAlAs quantum wells by low energy gallium focused ion beam implantation and thermal annealing, was characterized by low‐temperature photoluminescence spectroscopy. The depth extension of the induced disordering as well as the lateral resolution of interdiffused lines generated by line‐and‐space scan of the ion beam, was evaluated. On a multiple quantum well structure, no significant channeling of the ions, impinging on the target normally to the surface, could be evidenced, and the mixing was found to affect only the first quantum well located near the surface. On the other hand, the width of the interdiffused region observed laterally to the implanted lines is much larger than the spot diameter of the ion beam. It is concluded that the defects which activate the Ga–Al exchange at the interfaces, diffuse more rapidly in the aluminum rich layers. The influence of a rapid thermal annealing and the aluminum content in the barriers, on the lateral selectivity of the induced disordering is discussed.