This paper presents the results of temperature measurements inside the CanmetENERGY pilot scale entrained flow gasification facility by means of an optical sensor based on flame emission spectroscopy (FES). In the present work a cooled, purged, fibre optic probe inserted into the gasifier reactor, through an access port at one end and coupled to a small portable commercial spectrometer at the other end, collected radiation from the gasifier flame. The acquired spectra were analyzed to yield temperatures representing the reaction chamber region lying within an angle of 8.8 degrees enclosing the central axis of the probe. In the absence of a flame when radiation was acquired only from the hot enclosure, spectrally derived temperatures were in the same range as those recorded by wall mounted thermocouples (TE). When a flame was present spectral temperatures generally differed from the TE temperatures. Analysis of the spectra after the run yielded very high temperatures during certain periods of data acquisition indicating various types of gasifier burner malfunction. The thermocouples did not record similar high temperatures during any of the gasifier burner malfunction events. Examination of the gasifier after shut down, due to failure of a thermocouple, confirmed a malfunction had occurred during the operational period. These results show that FES derived temperatures are useful for monitoring gasifier performance. The data was further analyzed to explore the usefulness of other flame spectral features such as Na and OH emission for gasifier flame diagnostics. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.