The application of single-wavelength ellipsometry as an in situ technique for the simultaneous measurement of the silicon wafer substrate temperature and the surface oxide film thickness in a rapid-thermal processing system has been investigated. This technique is based on measuring the ellipsometry parameters ψ and Δ, and then calculating the wafer temperature and oxide thickness using polynomials derived from the known temperature dependence of the index of refraction of the silicon substrate and the silicon dioxide film. Temperature measured with ellipsometrey was compared to that measured with a thermocouple. The difference depends on the optical properties of the oxide and its thickness. For native-oxide-covered silicon wafers, the temperature measured by ellipsometry is within 20°C or less of that measured by a thermocouple for temperatures ranging from room temperature to 1000°C. For thicker oxides, similar results that agree with estimated error bounds were obtained. Temperature transients effects and technique optimization are discussed. © 1993, The Electrochemical Society, Inc. All rights reserved.