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Using steady-state fluorescence spectroscopy and UV-Vis absorbance spectrometry, the salient photophysical parameters of Congo red, a very important biological staining reagent, was determined. Its absorbance was observed at 497.0 nm; its molar absorptivity ε was determined as 6.26 × 104 M-cm. Its fluorescence, when excited at 330.0 nm, was observed at 417.0 nm. The quantum yield, ф, in aqueous solution was determined by two different methods—the relative, or comparative, method and the absolute method. Both methods gave the same value of 0.011. The room temperature fluorescence lifetime τo was determined as 2.8 ns using the Strickler-Berg equation. [ABSTRACT FROM AUTHOR]