A non destructive method to characterize the properties of SiO₂ films in SiO₂-Si structures and analyze their variations under different external actions is presented and discussed. The characterization of hole traps, electron traps and defect precursors states in SiO₂ films are carried out as well as their evolution and influence on the SiO₂-Si structure degradation behavior. Results measured on SiO₂-Si structures obtained from different technological processes and submitted to different treatments are reported in order to discuss the possibilities of this method as a power tool to assess the stability and quality of insulating layer on semiconductors.