© 2016 AIP Publishing LLC. We studied formation of the SiO2-T2O5 interface in the Si-SiO2-Ta2O5 structure using Cathodoluminescence Spectroscopy (CLS). Analyzing the evolution of CLS spectrum of the Si-SiO2 structure while depositing the Ta2O5 layer allowed to estimate an optical transmittance of the Ta2O5 layer and its band gap. Spectral features related to the formation of the SiO2-Ta2O5 interface were identified by comparison of the experimental CL spectrum of the Si-SiO2-Ta2O5 structure and its simulated counterpart. This formation involves a decomposition of silanol groups at the outer surface of the SO2 layer and creation of the SixTayO-type layer containing luminescence centers with the emission band centered at 3 eV photon energy.