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The method of species concentration measurements by laser-induced fluorescence (LIF) in flames was further developed. The applicability of the two-level rate-equation model was examined for the experimental conditions. Inhomogeneous saturation due to the spatial intensity distribution of the laser light was corrected for using a field of local laser intensities obtained experimentally. Among other things, the accuracy of the calibration of the optical detection system was crosschecked by two methods. Absolute number densities of CH and OH could be determined in a 13 mbar C2H2--O2 flame from measurements of the average saturation degree as a function of laser intensity. From these saturation curves approximate oscillator strengths could be obtained which provided a check of the evaluated number densities. Temperatures were determined from rotational spectra of the excited molecules. For the extension of the method to higher pressure, preliminary results obtained with a multilevel relaxation model are presented.