It is shown that high‐quality molecular‐beam epitaxial (MBE) regrowth of (Al,Ga)As on GaAs can be achieved by chemically passivating the GaAs surfaceexsituprior to regrowth with aqueous selenium reagents. Reflection high‐energy electron diffraction intensity oscillations show the bidimensional character of the regrowth and high‐resolution transmission electron microscopy reveals defect‐free regrown interfaces. Photoluminescence intensity from the Se‐treated GaAs surfaces on which Al0.5Ga0.5As is regrown rivals that from an allinsitugrown AlGaAs/GaAs interface. The high quality of these regrown interfaces could be attributed to the thermally and chemically stable selenium and oxygen phases that remain bound to GaAs under MBE conditions.