Diode-laser absorption sensors are advantageous because of their non-invasive nature, fast time response, and in situ measurement capability. New diode lasers, e.g., GaSb-based longer-wavelength (> 2) lasers and vertical-cavity surface-emitting lasers (VCSELs) have recentlyemerged. The objective of this thesis is to take advantage of these new diode lasers and develop new gas-sensing strategies.CO is an important species both as an air pollutant and a key indicator of combustion efficiency. Using (Al)InGaAsSb/GaSb diode lasers operating near 2.3 m, in situ measurements of CO concentration were recorded in both the exhaust (-470 K) and the immediate post-flame zone (1820-1975 K) of an atmospheric-pressure flat-flame burner. Using wavelength-modulation spectroscopy (WMS) techniques, a ~0.1 ppm-m detectivity of CO in the exhaust duct was achieved with a 0.4-s measurement time. For measurements in the iiunediate post-flame zone, quantitative measurements were obtained at fuel/air equivalence ratios down to 0.83 (366-ppm minimum CO concentration) with only an 11-cm beam path. These results enable many important applications, e.g.. ambient air quality monitoring, in situ combustion emission monitoring, and engine diagnostics.