Benzoporphyrin deriv. monoacid ring A [BPD-MA (I)], a chlorin-type mol., is a new photosensitizer currently in phase II clin. trials for the treatment by photodynamic therapy of cancerous lesions, psoriasis and pathol. neovascularization. The photochem. (type I and/or II) of I was studied in homogeneous soln. and in aq. dispersions of unilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) using ESR and spectrophotometric methods. When oxygen-satd. solns. of I were illuminated with 690 nm light, singlet oxygen (1O2), superoxide anion radical (O2.-), hydroxyl radical (.OH) and hydrogen peroxide (H2O2) were formed. I generates 1O2 with a quantum yield of about 0.81 in ethanolic soln. The quantum yield does not change upon incorporation of I into liposomes of DPPC. The superoxide anion radical was generated by the anion radical of I via electron transfer to oxygen, and this process was significantly enhanced by the presence of electron donors. The rate of prodn. of O2.- was also dependent on the concn. of I used (3-100 mM). The quantum yield of O2.- was found to be 0.011 and 0.025 in aq. soln. and DPPC liposomes, resp. Moreover, O2.- upon disproportionation can generate H2O2 and ultimately the highly reactive .OH via the Fenton reaction. In anaerobic homogeneous soln., radical anion of I was predominantly photoproduced via the self-electron transfer between the excited- and ground-state species. The presence of an electron donor significantly promotes the reduced form of I. These findings suggest that the photodynamic action of I may proceed via both type I and type II mechanisms.