Electron spin resonance study on heteroepitaxial Si/insulator structures obtained through the growth of epi-Lu2O3 films on (111)Si (∼4.5 % mismatched) by reactive molecular beam epitaxy indicates the presence in the as-grown state of interfacial Pb defects (∼5×1011cm-2) with an unpaired sp3 Si dangling bond (DB) along the  sample normal, prototypical of the standard thermal (111)Si/SiO2 interface. The defects, with density remaining unchanged to anneal in vacuum up to temperatures of Tan∼420 °C, directly reveal the nonperfect pseudoepitaxial nature of the interface, laid down in electrically detrimental interface traps. These are suggested to be interfacial Si DBs related to Si misfit dislocations. Alarmingly, defect passivation by standard anneal treatments in H2 fall short. For higher Tan, the interface deteriorates to “standard” Si/SiO2 properties, with an attendant appearance of EX centers indicating SiO2 growth. Above Tan∼1000 °C, the interface disintegrates altogether.