Data describing the deterioration of AlxGa1−xAs‐GaAs heterostructures in long‐term exposure (2–12 years) to normal room environmental conditions (∼20–25 °C, varying humidity) are presented. Optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron dispersion x‐ray spectroscopy are used to examine AlxGa1−xAs‐GaAs quantum‐well heterostructure material that has hydrolyzed at cleaved edges, cracks, and fissures, and at pinholes in cap layers. The hydrolysis is found to be significant for thicker (>0.1 &mgr;m) AlxGa1−xAs layers of higher composition (x>0.85).