Electron spin resonance analysis of (100)Si/LaAlO3 structures reveals the absence of a Si/SiO2-type interface in terms of archetypal Si-dangling bond-type Si/SiO2 interface defects (Pb0,Pb1). With no Pb-type defects observed, this state is found to persist during subsequent annealing (5% O2+N2 ambient) up to Tan∼800°C, indicating a thermally stable and abrupt Si/LaAlO3 interface. In the range Tan∼800–860°C, however, a Si/SiO2-type interface starts forming as evidenced by the appearance of Pb0 defects and, with some delay in Tan, the EX center (a SiO2 associated defect) attesting to significant structural/compositional modification. The peaking of the defect density versus Tan curves indicates that the interlayer with SiOx nature breaks up upon annealing at Tan≥930°C, possibly related to crystallization and silicate formation. No LaAlO3-specific point- - defects could be detected.