Spectroellipsometry (SE) has been used to investigate the silicon‐on‐insulator (SOI) structures formed by oxygen ion implanted into silicon and the corresponding annealed samples. A simple equation to estimate the thickness of the SOI structure is presented. Using a multilayer model and the Bruggeman effective medium approximation, we have analyzed the thicknesses and compositions of the SOI structures. Having been compared with the results determined by the Rutherford backscattering, spreading resistance probe, and infrared absorption measurements, our results show that, after high‐temperature annealing, the top layer of the SOI structure is close to a perfect single crystal silicon, and the buried SiO2 is in a disorder phase. The thicknesses determined by all these measurements are in good agreement with each other.