The optical functions of amorphous silicon thin films have been studied with spectroscopic ellipsometry (SE) in the temperature range from 290 to 520 K. The ellipsometry data were modeled using Tauc-Lorentz dispersion law for amorphous materials. It has been found that the temperature coefficients of Tauc-Lorentz parameters, such as the optical gap, are rather similar for four different materials, which suggests that the obtained values are valid for a broad range of amorphous silicon-based materials and can be used to determine the surface temperature by ellipsometry. A practical example of using spectroscopic ellipsometry for in situ temperature measurements in the plasma enhanced chemical vapor deposition environment is given. (C) 2004 Elsevier B.V. All rights reserved.