We describe two methods for producing thermally stable high resistivity layers in GaAs-AlGaAs heterostructures. These rely on the interaction of implanted ions with dopant impurities already present in a buried layer in the heterostructure. In the first case, oxygen implanted at a concentration above that of the acceptors in p-type GaAs is shown to create thermally stable, highresistivity material only in the case of Be-doping in the GaAs. The effect is not seen for Mg-, Znor Cd-doping. Similarly there is no apparent interaction of 0 with n-type dopants (S or Si). The Be-O complex in p-type GaAs is a deep donor, creating material whose sheet resistivity shows a thermal activation energy of 0.59 eV. In the second case oxygen implantation into n+ AlGaAs, followed by annealing above 600°C, creates a deep acceptor level that compensates the shallow donors in the material. Temperature dependent Hall measurements show the resistivity of the compensated AlGaAs has a thermal activation energy of 0.49 eV, in contrast to a value of 0.79 eV for non-induced damage compensation.