Sulfonated aluminum phthalocyanines interact with cytochrome c forming complexes in aqueous solutions. The binding of these dyes was probed using fluorescence techniques (steady-state spectroscopy and time-correlated single photon counting), which enable the calculation of the equilibrium binding constants. The fluorescence of the complex is relatively low (phi = 0.07) due to an electron-transfer process from the phthalocyanine singlet-excited state to the cytochrome c. The binding constant determines the amount of fluorescence and is quite high for the tetrasulfonated aluminum phthalocyanine (3.3 x 10(5) M-1) while those for the disulfonated and monosulfonated aluminum phthalocyanines are one order of magnitude smaller. The fluorescence decays suggest more than one location for the dye adsorbed in the protein, which is affected by electrostatic interactions, leading to nonexponential decays.