Aggregation of Fe-III-tetra-(4-sulfonatophenyl)-porphyrin ((FeTPPS)-T-III) was studied in the presence of block copolymers, poly(ethylene glycol)-block-poly(4-vinylpyridine) (PEG-b-P4VP), poly(ethylene glycol)-block-poly(2-(dimethylamino)ethyl methylacrylate) (PEG-b-PDMAEMA), and poly(ethylene glycol)-block-poly(beta-cyclodextrin) (PEG-b-PCD). The interaction between the iron porphyrin and the blocks, P4VP, PDMAEMA, and PCD, led to the formation of copolymers/(FeTPPS)-T-III complex micelles with a PEG shell and determined the species of (FeTPPS)-T-III. The electrostatic interaction of protonated P4VP and PDMAEMA with (FeTPPS)-T-III remarkably decreased the apparent pK(d) of (FeTPPS)-T-III and led to a micellar mu-oxo dimer of the iron porphyrin. At pH above the pK(a) of P4VP, (FeTPPS)-T-III was kept inside the hydrophobic P4VP core and formed an encapsulated mu-oxo dimer. However, when above the pK(a) of PDMAEMA, (FeTPPS)-T-III escaped from the hydrophobic PDMAEMA core, existing as a free mu-oxo dimer. PCD caused the monomer of the porphyrin because of the inclusion complexation between the beta-cyclodextrin residues and (FeTPPS)-T-III. The two micellar monomer species (FeTPPS)-T-III(H2O)(2) and (FeTPPS)-T-III(OH) were obtained with an equilibrium pK(a) similar to 6.4.