Interaction of the dye Congo red (CR) with fibrils of three model proteins—hen egg lysozyme, recombinant human beta 2-microglobulin (b2M), and recombinant human transthyretin (TTR)—has been investigated using spectrophotometry. Considerable amounts of impurities were detected in the commercial dye formulation. A procedure of dye purification has been developed. The molar extinction coefficient of the dye at 490 nm (ε 490 ) has been measured; the coefficient was 3.3 × 10 4 M –1 cm –1 at pH > 6.0. The formation of a complex between CR and the fibrils was accompanied by a change in the absorption spectrum of the dye in the visible wavelength range. Titration of fibril solutions with excessive amounts of dye showed that the number of CR molecules bound to a protein monomer within the lysozyme fibrils was close to five, whereas the respective ratio for b2M was close to four, and the ratio for TTR fibrils was close to four molecules per protein subunit.