Microstructure of complicated glasses in the system 30Na 2 O-2Al 2 O 3 -25 SiO 2 -xCeO 2 (43-x) B 2 O 3 , x changes from 0.5 to 20 mol% have been extensively studied. Structural determination of glasses containing high cerium oxide content (≥8 mol% CeO 2 ) was carried out by 11 B NMR and FTIR spectroscopy. On the other hand, 29 Si MAS NMR experiment is hardly to be applied to glasses of CeO 2 > 8 mol%. This is due to the paramagnetic action which is raised by cerium cations causing dilution or delaying in the resonance phenomenon. It is evidenced from NMR data that sodium oxide is high enough to modify the glass forming units which constitute the skeleton of the glass. Ceria is as well as silica and B 2 O 3 all are acting as glass forming species. Decreasing of both fraction of boron tetrahedral units (N 4 ) and chemical shift of silicon nuclei (δ) confirm the role of CeO 2 as a glass former. On the other hand, fast decrease in N 4 and chemical shift of Si nuclei with further increasing CeO 2 contents (≥8 mol%) gives a clear evidence that the ability of cerium oxide to participate as a network former increases with increasing its content. New approach is applied to determine the fraction of CeO 4 as a glass forming units. In this approach, we use the common advantage of 11 B NMR and FTIR spectroscopy to obtain Ce 4 fraction. The latter species cannot be determined from NMR spectroscopy, since very high relaxation time and magnetization of ceria cause intensive spectral broadening which prevent resonance spectra to be appeared.