The apparatus for measuring concentrations of fuel mixtures using depth-resolved laser-induced fluorescence is a fluorometer equipped with a sample container holder that is movable in the path of the beam from the light source. Fluorescent emissions from the sample mixture pass at 90° to the excitation light path through a slit that is narrow enough that the emission intensity is effectively produced by a thin layer of the sample and focused on a monochromator, with successive thin layers receiving nonuniform excitation radiation due to reduction of intensity along the excitation light source path with increasing depth penetration and due to reabsorption of emitted fluorescence from adjacent layers. The method has a first mode in which the emission spectrum is scanned at a fixed depth, and a second mode in which the sample is moved relative to the emission monochromator slit to vary the depth while keeping the emission wavelength fixed.