Transmission electron microscopy (TEM) and photoluminescence (PL) data are presented on GaAs/Ga0.25Al0.75As quantum well (QW) structures grown by molecular beam epitaxy which were implanted with B ions and Si ions, respectively, to a dose of 1015 cm-2. TEM observations reveal that significant intermixing of the layers occurs in Si implanted QW structures at a depth well beyond the projected range of the implanted ion after 1 h thermal annealing at 850 °C. A subsequent 2‐h annealing causes mixing near the surface, while the unmixed region still remains at a distance twice the projection range. In contrast, intermixing was not observed in annealed B implanted QW structures, which suggests a strong dependence of alloy mixing effects on the impurity. PL data support these results.