Crystalline Si wafers oriented in the <100> and <111> planes were oxidized in ambients containing low oxygen partial pressures to examine the role played by oxidant pressure on relative oxidation rates. The grown oxide films were formed at temperatures between 850° and 1050°C and were less than 300A thick. Substrates with <111> oriented surfaces are generally oxidized more readily than substrates with <100> orientations at the same oxidation temperature and oxidant pressure. However, a reversal in the relative oxidation rates of these crystalline Si substrates was observed at low oxygen partial pressures. This reversal was nearly independent of oxidation temperature and also of oxidant diluent gas. A qualitative model is proposed to account for these results. © 1980, The Electrochemical Society, Inc. All rights reserved.