Ellipsometric analysis of surface SiO2 on Separation by IMplanted OXygen (SIMOX) wafers has been performed. Spectroscopic ellipsometry data in the range of 500-850 nm were fitted based on a structure model composed of surface SiO2, surface Si, and buried SiO2 layers as well as transition layers at the SiO2/Si interfaces. From the fitting results, it is found that there exist transition layers with a thickness of 1.6 and 2.0 Angstrom at the surface SiO2/surface Si and buried SiO2/Si interfaces, respectively. Angle-resolved He-Ne laser ellipsometry has been also applied for in-situ monitoring of oxidation processes of SIMOX wafers and it is shown that ultrathin oxide thickness can be determined with an accuracy of 3 Angstrom.