We report on the use of a commercially available band-edge detection system for substrate temperature monitoring of gallium arsenide substrates. The extension of the technique to the cases where strong absorption by either the substrate or substrate holder might normally preclude the use of such systems due to poor signal levels is discussed. For indium-mounted wafers, a background subtraction/removal is applied which allows unambiguous determination of the band edge across the full temperature range. An alternative method of operation of the instrument as a highly configurable pyrometer allows measurements to be made on highly conducting p-type substrates where free carrier absorption swamps the band edge. (c) 2007 Elsevier B.V. All rights reserved.