The effect of ultrasonic treatment (UST) on the defect structure of the Si-SiO2 system is characterised by means of electron spin resonance (ESR), metallography, MOS capacitance measurements and secondary ion mass spectroscopy (SIMS). A non-monotonous dependence of the defect densities on the ultrasonic wave intensity has been observed. The influence of the UST frequency on the ESR signal intensity of the defect centres depended on the defect's type and structure and may be caused by vibrational energy dissipation which is a function of the defect centre's type. The influence of the UST on the Si-SiO2 interface properties depends on the oxide thickness and crystallographic orientation. The density of point defects and absorbed impurities at the Si-SiO2 interface can be reduced and its electrical parameters improved by an appropriate choice of UST and oxidation conditions.