Optical measurements of substrate temperature during the growth by plasma-assisted molecular beam epitaxy of GaN on sapphire and GaAs (1 1 1)B were obtained using two commercial BandiT systems for visible and infrared wavelengths. With the visible system, an absorption edge at similar to 400 mit was seen after 20-50 nm of GaN growth, and although this became clearer and more stable for the film on sapphire, strong interference oscillations precluded further measurements on the GaAs substrate. Using the infrared version, we observed a real initial increase in temperature for GaN films grown on GaAs substrates followed by a subsequent decrease to a steady-state value. Crown Copyright (c) 2007 Published by Elsevier B.V. All rights reserved.