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The fluorescence ratio technique for processing planar laser-induced fluorescence data was generalized for quantitative imaging of the injectant mole fraction and extended to quantify the density distributions in a nonreacting supersonic mixing flowfield. The original fluorescence ratio approach was first developed by Hartfield et al. (Hartfield, R. J., Jr., Abbitt, J. D., III, and McDaniel, J. C., 'Injectant Mole Fraction Imaging in Compressible Mixing Flow Using Planar Laser-Induced Iodine Fluorescence,' Optics Letters, Vol. 1, No. 16, Aug. 1989, pp. 850-852.) for tests in a special closed-loop wind tunnel to eliminate the effects of thermodynamic property variations on planar laser-induced fluorescence signals in compressible flowfields. This approach provided us a quantitative means of planar mole-fraction measurement; however, it implicitly assumed that the tracer molecules were seeded at the same fraction in both the main and the secondary flows. In the present study, we generalized the Hartfield et al. method by considering differences in the tracer-seeding rates for obtaining planar images of mole fraction and density. Experimental validation of the new method was carried out in a mixing flowfield formed by sonic transverse injection into a Mach 1.9 supersonic airstream. The injectant mole-fraction distribution obtained from planar laser- induced fluorescence data processed by our new approach showed better agreement with the gas-sampling data than one based on the Hartfield et al. method. The density distribution was verified by comparison with the theoretical density ratio across the oblique shock wave. DOI: 10.2514/1.37783