Effects of annealing on oxygen depth profiles and chemical etching rates of thermally grown silicon oxides are investigated in order to characterize the intermediate layer, adjacent to the oxide/silicon interface. We found that the oxygen Auger signal intensity for the samples grown at 800 and 900 degrees C with a thickness of 11 nm, increases in the region that is several nanometers thick, adjacent to the oxide/silicon interface. Furthermore, the chemical etching rates of the samples grown at 800 and 900 degrees C decrease in just about the same region, adjacent to the interface. However, we found no intermediate layer in the samples grown at 1000 degrees C, or annealed at 1000 degrees C. These experimental results indicate the existence of an intermediate layer that has a higher oxygen content, and dissolves slower in an etching solution, than the bulk oxide. The intermediate layer is believed to contain highly dense quasi-stable silica that is compacted by the compression stress. Annealing at 1000 degrees C must result in a transformation of the intermediate layer to a more stable structure with a lower density.