Based upon the linear-parabolic growth model of silicon oxidation, accurate kinetic rate constants are determined for (100), (111), and (110) silicon oxidized in dry oxygen in the 800°-1000°C range. The oxide growth was monitored by high temperature automated in situ ellipsometry. It is shown that fitting the maximum number of oxidation data points to a linear-parabolic relationship yields accurate oxidation rate constants that are unique to the oxidation process as described in the Deal-Grove model, and not just good empirical fitting parameters. This approach is denoted the “optimum Xi technique.” Both linear and parabolic rate constants exhibit a break in their activation energies at 950°C. This behavior is discussed and interpreted in terms of the viscoelastic properties of SiO2. © 1985, The Electrochemical Society, Inc. All rights reserved.