In this paper we report a novel technique, based on optical measurements. to measure the true temperature of the wafer surface during epitaxial growth. It will be shown that this temperature can deviate considerably from the susceptor temperature measured with thermocouples or pyrometers. For this purpose we employed combined in-situ reflectance anisotropy spectroscopy and spectroscopic reflectance measurements in a number of different metal-organic vapor phase epitaxy (MOVPE) reactors. Measurements have been performed on rotating and non-rotating samples during growth of GaAs, AlAs and AlxGa1-xAs. We demonstrate that in a single growth run the reading of a conventional thermocouple can be calibrated to the true wafer temperature. the growth rate can be determined and process calibration for the AlGaAs composition can be established. (C) 2002 Elsevier Science B.V. All rights reserved.