We describe electrical and spectroscopic measurements of SIMOX subjected to supplemental oxygen implantation and standard 1000° C post implantation annealing. Point contact transistor measurements indicate that the supplemental oxygen implantation creates a net increase in radiation-induced trapped charge, and, surprisingly, electron paramagnetic resonance studies show that the O vacancy concentration increases. The electron paramagnetic resonance results suggest that the radiation tolerance of the supplemental oxides is due to insufficiently annealed implantation damage. A higher post supplemental anneal temperature minimizes the radiation-induced trapped charge, but further studies are necessary to assess the total impact of the high temperature anneal. © 1994 IEEE