The influence of supplemental O+ implantation on standard separation-by-implantation of oxygen (SIMOX) structures in terms of inherent defects has been analyzed by electron spin resonance. The monitored defects include the Eγ′ and Eδ′ variants of the oxygen-deficiency centers in the buried oxide layer (BOX), and the likely O-related, shallow donor (UL1) centers in buried interfacial Si layers. E′ generation sensitivity depth profiles were mapped using etch back in combination with E′ generation by exposure to a dc Ar glow discharge. A major finding is that supplemental O implantation significantly reduces the E′ generation sensitivity (precursor sites) quite uniformly all over the BOX layer. The reduction, though, is about two times greater for Eδ′ as compared to Eγ′. The UL1 centers, by contrast, are seen to increase by up to a factor 6. Apparently these defects, inherent to the SIMOX formation process, are insufficiently removed by the postsupplemental O+ implantation anneal performed at 1100 °C.