Implantation of 1.8×1018O+/cm2 into silicon results in a buried oxide (BOX) layer, nominally 400 nm thick. The as-implanted BOX has ∼1020 cm−3 dangling bonds, ∼1020cm−3 reactive sites for interaction with H (D); the dielectric properties are ill-defined, and, similarly to densified or ion-bombarded silica glass, the SiO2 network contains an increased proportion of high energy Si-O bonds, i.e. it is “strained”. After annealing above 1300°C the properties approach those of thermally grown SiO2, the dangling Si bonds are eliminated, the D-uptake decreases, the Si/SiO2 interfaces become sharper, and the dispersed silicon agglomerates into clusters of various kind. However, the properties of BOX are still significantly different from those of thermally grown SiO2 films as shown by pronounced electron trapping, large concentration of different hole traps, decreased etch rate in HF, increased D-uptake and E' center generation rate, as well as by increased bulk conduction and the presence of localized conducting defects. These phenomena are likely to be related to some oxygen deficiency, e.g. O3SiO3 groups, as oxygen treatments reduce, but not completely eliminate, the difference between BOX and thermal oxide.