AlGaN/GaN microwave HFET layer structures are grown on sapphire and semi-insulating SiC substrates by MOVPE at substrate temperatures in the range 1000-1100degreesC. Growth conditions such as growth rate and effective V, III ratio are substrate temperature dependent, as are many Of the ultimate device characteristics. A reliable method of measuring the actual substrate temperature has yet to be established. Direct Measurement of the substrate temperature in an MOVPE chamber is complicated by the following issues: (i) decoupling of the thermocouple from the substrates (ii) changes in substrate emissivity during heating and growth (iii) gradual coating of tile chamber and optical ports causing a drift in thermal properties and pyrometer reading. We report a method of referencing the substrate temperature. which avoids these problems by monitoring the shift in the absorption edge of a SiC substrate using in situ spectral reflectometry. This technique enables the actual substrate temperature to be reproduced to within 10degreesC run to run. (C) 2002 Elsevier Science B.V. All rights reserved.