Microscopic voids, formed from the condensation of supersaturated vacancy point defects, were recently discovered in implanted and annealed GaAs. These defects have been shown to suppress carrier concentrations. Since voids are formed only at relatively high temperatures (> 650 °C), the possibility exists that voids can be used for thermally stable implant isolation. In this paper, we report on the formation of highly resistive layers in GaAs, created by Al+ implantation and annealing in the 700–900 °C range. In samples containing voids, their sheet resistivities increased by about six orders of magnitude from the as-grown value. Formation of these thermally stable, high resistivity regions is different from the conventional H or O implant isolation techniques, which use lattice damage to create the isolation characteristics. However, since lattice damage is annealed out between 400–700 °C, this type of isolation becomes ineffective at high processing temperatures. By contrast, voids are stable at high processing temperatures, and potential advantages of using such defects for device isolation in GaAs are pointed out.