The absorption spectra of bilirubins were deconvoluted by two Gaussian curves of equal width representing the exciton bands of the non-degenerate molecular system. The two bands were used to study the wavelength dependence of the (4Z, 15Z) → (4Z, 15E) configurational photoisomerization quantum yield of the bichromophoric bilirubin-IXα (BR-IX), the intrinsically asymmetric bile pigment associated with jaundice and the symmetrically substituted bilirubins (bilirubin-IIIα and mesobilirubin-XIIIα), when they are irradiated in aqueous solution bound to human serum albumin (HSA). The same study was performed for BR-IX in ammoniacal methanol solution (NH₄OH/MeOH). The quantum yields of the configurational photoprocesses were fitted with a combination function of the two Gaussian bands normalized to the total absorption, using the proportionality coefficients and a scaling factor as parameters. The decrease of the (4Z, 15Z) → (4Z, 15E) quantum yield with increasing wavelength, which occurs for wavelengths longer than the most probable Franck–Condon transition of the molecule, did not result in a unique function of the exciton absorptions. In particular we found two ranges corresponding to different exciton interactions with different proportionality coefficients and scaling factors. The wavelength-dependent photoisomerization of bilirubins was described as an abrupt change in quantum yield as soon as the resulting excitation was strongly localized in each chromophore. The change was correlated to a variation of the interaction between the two chromophores when the short-wavelength exciton absorption became vanishingly small. With the help of the circular dichroism (CD) spectrum of BR-IX in HSA, a small band was resolved in the bilirubin absorption spectrum, delivering part of the energy required for the (4Z, 15Z) → (4Z, 15E) photoisomerization of the molecule.