Low temperature electron spin resonance (ESR) measurements were carried out on a broad range of separation by implantation of oxygen (SIMOX) substrates. The post-implantation forming anneal (T 1325°C) was observed to induce a donor in the SIMOX structure, more specifically, a shallow effective mass donor in Si of axial symmetry and preferential alignment along , likely an oxygen-related heat-treatment donor. Controlled etching back experiments revealed that the donor defects reside in an interfacial Si layer of at least 600$$A thick at both sides of the buried oxide layer reaching a local volume density of about 1018 cm−3. In the flat band situation, the donor was observed to be in a diamagnetic, ESR non-active state. Downward sweeping of the Si energy bands near the buried oxide interfaces, relative to the Fermi level, either by in situ positive ate biasing of large area metal-oxide-silicon capacitors fabricated on the buried oxide or γ7-irradiation induced positive charging of the buried oxide, was observed to alter the ionization state of part of the donors into an ESR-active state.