Abstract—The absorption and fluorescence properties of porphyrinc(Pc), the porphyrin chromophore present in cytochromec, have been determined in several solvents and micellar environments. In aqueous buffer solutions at pH 7.5 Pc may exist in both a fluorescent monomeric form with quantum yield of fluorescence, (Φf,) ∼ 0.03, and fluorescence lifetime, (τf) ∼ 8 ns, and as a non‐fluorescent aggregate. The proportion of monomeric form is higher in organic solvents and micelles but is reduced with increasing porphyrin concentrations in aqueous solutions. Porphyrincreadily complexes with Zn2+to produce a fluorescent chelate (Zn‐Pc) with Φf, ∼ 0.02 and τf, ∼ 2 ns at pH 7.5. The yields of singlet excited oxygen formation from Pc and the Zn‐Pccomplex are higher than observed for hematoporphyrin derivative (HpD). Both Pcand Zn‐Pcare effective agents in tumor phototherapy and do not induce the prolonged cutaneous photosensitivity observe