The atomic nature of the interface in (100)Si/LaAlO3 structures with nanometer-thin amorphous LaAlO3 layers of high dielectric constant (κ), deposited directly on clean (100)Si by molecular beam deposition at ∼100°C, was assessed through probing of paramagnetic point defects. On the as-grown samples K-band electron spin resonance indicated the absence of a Si/SiO2-type interface in terms of the 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 (1atmN2 or 5% O2 in N2 ambient) up to the temperature Tan∼800°C, referring to a thermally stable abrupt Si/LaAlO3 interface, quite in contrast with other high-κ metal oxide/Si structures. However, in the range Tan∼800–860°C a Si/SiO2-type interface starts forming as evidenced by the appearance of Pb0 defects and, with some delay in Ta- n, the EX center—a SiO2 associated defect, attesting to significant structural/compositional modification. The peaking of the defect density versus Tan curves indicates the SiOx nature of the interlayer to break up again upon annealing at Tan≥930°C, possibly related to crystallization and/or degrading silicate formation. No specific LaAlO3-specific point defects could be traced.