An in situ thermometry technique was developed for low-pressure flames based on laser absorption spectroscopy of nascent carbon dioxide. Discrete, rovibrational lines within the v3 fundamental band near 4.2 mu m were utilized due to their superior strength, large temperature sensitivity and isolation from interfering absorption lines of other combustion species. These transitions were simultaneously accessed with a single interband cascade laser using scanned-wavelength direct absorption and Voigt lineshape fitting to infer temperature from the ratio of integrated absorbance. A single-ended optical probe constructed of sapphire rods was used to deliver the laser light to the flame. Measurements of temperature and CO2 were conducted in burner-stabilized flames for methane-oxygen and ethylene-oxygen reactant mixtures over a pressure range of 25-60 torr and from a distance of 3-23 mm above the burner face. Measurements were compared to a 1-D kinetic model as well as thermocouple measurements. The sensor was also used for path-averaged thermometry on a propane torch at 760 torr to demonstrate the potential for measurement at atmospheric pressure. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.