For developing clinically useful porphyrin drugs, it is essential to characterize porphyrin-membrane interactions and to determine the factors that modulate such interactions. To this end, four uniquely p-phenyl-substituted tetraphenylporphyrins were synthesized. These water-insoluble, unsymmetrically substituted porphyrins were allowed to diffuse into aqueous micellar solutions formed by the surfactants tetradecyltrimethylammonium bromide (TTAB), sodium dodecyl sulfate (SDS), and poly(ethylene glycol)p-t-octylphenol (TX-100). The abilities of the porphyrins to localize in these micelles were determined by UV-vis and NMR spectroscopy. The data show that the NO2-phenyl-substituted porphyrin did not diffuse into any of the micellar solutions. The COO --substituted porphyrin was solubilized in cationic TTAB and in nonionic TX-100 micellar solutions under neutral and basic conditions. The NH3 + -substituted porphyrin incorporated in anionic SDS micelles at pH ) 2 and in TX-100 micelles at pH 2 and 7. These results emphasize that charge and polarity of the porphyrin substituent and its electrostatic interactions with the micelles play important roles in incorporating porphyrins with charged substituents into micelles. The OH-phenyl-substituted porphyrin incorporated into both neutral and basic TTAB and TX-100 micellar solutions in the highest concentrations, which reveals that a hydroxy substituent placed at the porphyrin periphery significantly increases the tendency of the porphyrin to embed in cationic and nonionic micelles. The data further demonstrate that all porphyrins are monodispersed in given micelles. In terms of porphyrin location, the data suggest that the COO --and NH3 + -phenyl-substituted porphyrins localize in the hydrophobic interior of ionic micelles, whereas the OH-phenyl-substituted porphyrin adopts a location in the more polar domains of cationic micelles. In nonionic micelles, the COO --, NH3 + -, and OHphenyl-substituted porphyrins seem to orient themselves toward the water-micelle interface. An intercalation among the surfactant chains is proposed.