ent spectra (maxima of absorption, circular and linear dichroism) of individual chromophores have been assigned for phycoerythrocyanin (PEC) trimer, monomer(s), and its subunits (α-PEC and β-PEC) by titration with p-chloromercury-benzene-sulfonate (PCMS), linear dichroism and photochemical transformations, as well as by deconvolution using a‘bilin’ line-shape spectrum based on the α-84 phycoviolobilin-chromophore in the α-subunit. The level ordering PVB-α-84→PCB-α-155→PCB-α-84 is the same irrespective of aggregation. Two different monomers (αβ) were observed. In 4M urea, the spectra are appropriately weighted sums of the subunit spectra, whereas in the monomer obtained in 1M KSCN, both β-chromophores are red-shifted by 4-5 nm. Formation of trimer (αβ)3 gives considerable spectral changes: (1) the absorption is narrowed, which has been rationalized by excitonic coupling between neighbouring monomers, (2) the short wavelength part in the CD spectrum is missing and (3) a fourth band (+) at 528 in the LD spectrum appears. A deconvolution of the trimeric aggregation state using only the ‘bilin’ line-shape model is not possible.