A multiplexed diode-laser sensor system based on second harmonic detection of wavelength modulation spectroscopy (WMS) is developed for application at elevated temperatures with two near-infrared diode lasers multiplexed using a frequency-division multiplexing scheme. One laser is tuned over a H<sub>2</sub>O line pair near 7079.176 and 7079.855 cm<sup>?1</sup>, and another laser is tuned over a pair of CO<sub>2</sub> and CO lines near 6361.250 and 6361.344 cm<sup>?1</sup>. Temperature and concentrations of H<sub>2</sub>O, CO<sub>2</sub>, and CO could be measured simultaneously by this system. In order to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling, the WMS-1<i>f</i> normalized 2<i>f</i> method is used. Demonstration experiments are conducted in a heated static cell. The precision of temperature and the concentrations for H<sub>2</sub>O, CO<sub>2</sub>, and CO are found to be 1.57%, 3.87%, 3.01%, and 3.58%, respectively. These results illustrate the potential of this sensor for applications at high temperatures.

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