The authors explore E′ trapping centers in separation by implanted oxygen (SIMOX) buried oxides with electron spin resonance (ESR) and capacitance vs. voltage (CV) measurements. Through the use of vacuum-ultraviolet (VUV) (hc/ = 10.2 eV) and ultraviolet (hc/ = 5 eV) illumination combined with ESR and CV measurements, they present evidence that E′ centers are important in SIMOX trapping and that thermal oxide trapping and SIMOX trapping involve different mechanisms. It is shown that E′ precursors are present in higher density in the SIMOX oxides explored than in thermal oxides. Large changes in E′ density are induced by injecting electrons or holes into VUV illuminated oxides; this shows that a high percentage of the centers are efficient traps. CV measurements show low amounts of net space charge; this suggests a compensating trap mechanism.