The application of ellipsometry as a technique for in situ silicon wafer temperature measurement in a rapid thermal processing environment was investigated. This technique is based on the ellipsometric measurement of the refractive index of silicon, and then the determination of the temperature from the known temperature dependence of the refractive index. An algorithm was developed which enables the determination of the resolution limits of this technique. Within the temperature range from 0 to 1100°C, a worst–case temperature error of ± 10°C can be expected for an ellipsometer operating at a wavelength of 6328 Åand resolving 0.01° in the measured parameters Ψ and Δ, and in the angle of incidence ϕ For an ellipsometer operating at a wavelength of 4133 Å, the maximum error improves to within ± 1.4°C, for the temperature range investigated from 0 to 700°C. © 1993, by The Electrochemical Society, Inc. All rights reserved.