The interaction of a zinc tetrasulfonated phthalocyanine with cytochrome c was studied using steady-state spectroscopic techniques and time-correlated single photon counting in water and Triton-X 100 micelles. The dye forms dimers in water with a high equilibrium constant (70 x 10(6) M-1). Because of a specific electrostatic interaction, the presence of cytochrome c does not lead to a dissociation of this dimer, but increases its formation, with an equilibrium constant of about 7.9 x 10(9) M-1. Triton-X 100 micelles dissociate the dimer, creating two populations of dye molecules: one in a hydrophilic media, probably on the surface of the micelles, another on a hydrophobic environment, probably inside the micelles. However, when cytochrome c is added the dye aggregation is again induced leading to a strong fluorescence quenching. This fluorescence quenching may also be caused by a photoinduced electron-transfer due to the formation of a 1:1 complex between the dye and the protein, but the present work does not give direct evidence of such an effect because the fluorescence decays did not show the presence of an extra component. The results presented here are quite different from those reported for aluminum sulfonated phthalocyanines, where aggregation does not occur and the fluorescence quenching is solely due to photoinduced electron-transfer reactions.