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A diode laser system was utilized to obtain spatially and temporally resolved absorption spectra during flame spread through non-homogenous fuel-air mixtures. The diode laser was wavelength-modulated in order to increase detection sensitiviy of the system. Gas concentrations were determined from the absorption spectra using regression models.Two fundamentally different systems were studied in this work. Methane concentrations were determined from absorption measurements in a buoyant plume of methane and air formed in a vertical low speed flow tunnel. Methanol and water concentrations were determined from absorption measurements in a system in which a non-uniform methanol-air layer was formed by evaporation along a horizontal gallery floor. In both systems, the layered mixture was ignited and measurements were obtained as the flame propagated. Measurements in the gallery were made in both terrestrial and microgravity environments. Thus, the ability to measure gas concentration using a system that meets the severe restrictions of microgravity research was demonstrated.