The initial stage of the thermal oxidation of various crystallographic orientations of silicon ((100), (110), and (111) orientations) reveals a complex rate behavior. This behavior is not understood within the conventional linear‐parabolic model. A recently revised model which explicitly contains the areal density of Si atoms and mechanical stress effects is shown to provide both a qualitative (for all orientations studied) and somewhat quantitative (for (110) and (111) orientations) explanation of the complex substrate orientation effects.