Quantum well (QW) intermixing using high-energy ion implantation is a promising technique for laterally selective, post-growth modification of a quantum well structure. In this work, we investigate the lateral selectivity of the technique, which is a function of the ion straggling during implantation and of the lateral diffusion of defects during post-implantation annealing. We have used photoluminescence and a specially designed mask to monitor the intermixing of QWs under masked regions. A significant amount of intermixing, resulting in a blueshift of the QW band-gap energy, was observed when the mask stripe width was less than 5 mu m, thus giving a lateral selectivity of 2.5 mu m.