Fully isolated islands have been formed in silicon using selective, lateral thermal oxidation at the base of 250-nm-wide structures. The final structure consists of substrate silicon on thermal oxide on substrate silicon. The process begins with the definition of 250-nm-wide islands that are capped on the top and sidewall with a silicon dioxide/silicon nitride oxidation mask. The structure is then isotropically or anisotropically recess etched and thermally oxidized to produce isolated silicon islands. Our experiments show that the quality of the silicon-on-insulator structure depends on the oxidation mask, the island dimension, the profile of the recess etch, and the oxidation time and temperature. The degree of isolation can be tailored by controlling the lateral oxidation of the filament connecting the island to the underlying substrate. By this technique we have formed silicon filaments of 10 to 100 nm in width.