A method for the calculation of absolute fluorescence quantum yields for dyes attached to solid particles based on reflectance measurements is reported. The same procedure allows calculation of true reflectance spectra (free of fluorescence) for highly fluorescent materials as well. Samples of cresyl violet were immobilized by adsorption on microgranular cellulose in the concentration range 4.5 x 10(-9) to 3.8 x 10(-6) mol g(-1). Diffuse and total reflectance spectra were recorded with and without insertion of an optical absorption filter between the output of the integrating sphere of a reflectance spectrometer and the photodetector in order to block fluorescence partially. From these data, the relative emission spectrum of the dye, the filter transmission spectrum, and the detector sensitivity, true reflectances and absolute fluorescence quantum yields were recovered. Observed fluorescence quantum yields, affected by dye aggregation and inner filter effects, were concentration and wavelength dependent, ranging approximately between 0.1 and 0.6. The analysis of remission function spectra showed that dye aggregation is negligible up to a concentration of 1.41 x 10(-7) mol g(-1). Fluorescence data were corrected for reemission and reabsorption using a suitable model [Lagorio, M. G.; Dicelio, L. E.; Litter, M. I.; San Roman, E. J. Chem. Soc., Faraday Trans. 1998, 94, 419]. Application of this model to samples showing no aggregation yielded a wavelength-independent true fluorescence quantum yield of 0.60 +/- 0.05, similar to values found in solution. The usage of cresyl violet as a reference for the evaluation of fluorescence quantum yields for weakly fluorescing samples in the solid phase is discussed.