This study is concerned with the initial regime (i.e. up to 200Å thickness) of the thermal oxidation of single crystal Si. The kinetic data ( film thickness vs. time of oxidation) has been generated at 780°, 893°, and 980°C in dry and ambients and measured using an automated ellipsometer which follows the oxidation in situ. The data for the dry oxidation is considerably more linear than for the data produced from containing ambients which exhibit parabolic behavior. This may be attributed to a greater protectiveness for oxides grown in . Dielectric breakdown measurements show that there are fewer defects in the grown thin films. Transmission electron microscopy observations show that the films contain inhomogeneities which are smaller than 50Å. Therefore, the conclusion is that the pore structure is different in dry and wet grown films and that this microstructure is responsible for both the oxidation data and dielectric breakdown differences.