A diode-laser sensor system based on absorption spectroscopy techniques has been developed to measure CO, CO 2 , and H 2 O concentrations and gas temperature non-intrusively in the combustion and exhaust regions above a C 2 H 4 -air burner operating at atmospheric pressure. In situ measurements of CO concentration in the combustion region (1.5 cm above the burner) and in the exhaust (79 cm above the burner) were determined from absorption lineshapes of the R(30) and R( 15) transitions (2m band) near 2.3 lm, respectively, using new research-grade GaSb-based diode lasers. A minimum detectivity of less than 10 ppm was demonstrated in the exhaust region. In situ measurements of H 2 O concentration and gas temperature were determined from absorption lineshapes near 1.343 lm (m 1 โซืโฌm 3 band), 1.392 lm (2m 1 , m 1 โซืโฌm 3 bands), and 1.799 lm (2m 2 โซืโฌm 3 โ m 2 band) using distributed feedback (DFB) diode lasers. In situ measurements of CO 2 were recorded using a DFB laser near 1.997 lm (R [50]) transition, m 1 โซ2ืโฌm 2 โซืโฌm 3 band). Gas temperature and H 2 O and CO 2 concentrations were monitored simultaneously along a single path in the combustion region (1.5 cm above the burner surface) using a wavelength-multiplexing arrangement. The CO, CO 2 , and H 2 O concentration measurements in the combustion region agreed with calculated equilibrium populations to within the experimental uncertainty (10% for CO 2 and H 2 O, 5% for CO), and the measured temperatures were in agreement with radiation-corrected type-S thermocouple values to within 4%. These results demonstrate the utility of diode laser sensors for fast in situ measurements of multiple important combustion parameters for emissions-monitoring and closed-loop control applications.