The new block amphiphilic copolymer, poly(sodium styrenesulfonate-block-5-(4-acryloyloxyphenyl)-10,15,20-tritolylporphyrin) (PSSS-block-Po), was synthesized by nitroxide-mediated "living" radical polymerization. The polymer is soluble in water and can absorb light in the visible region. The absorbed energy migrates between the porphyrin (Po) polymeric chromophores. The polymer chains form nanospheres in aqueous solution. Their interiors are highly hydrophobic and can effectively solubilize the large hydrophobic molecules of the various organic compounds (pyrene, azulene, rubrene). The polymer photosensitizes oxidation of organic compounds via photoinduced electron transfer and/or via formation of singlet oxygen.