The photophysical properties of benzoporphyrin derivative monoacid ring A (BPD-MA), a second-generation photosensitizer currently in phase II clinical trials, were investigated in homogeneous solution. Absorption, fluorescence, triplet-state, singlet oxygen (O2 (1 delta g)) sensitization studies and photobleaching experiments are reported. The ground state of this chlorin-type molecule shows a strong absorbance in the red (lambda approximately 688 nm, epsilon approximately 33,000 M-1 cm-1 in organic solvents). For the singlet excited state the following data were determined in methanol: energy level, Es = 42.1 kcal mol-1, lifetime, tau f = 5.2 ns and fluorescence quantum yield, phi f = 0.05 in air-saturated solution. The triplet state of BPD-MA has a lifetime, tau T > or = 25 microseconds, an energy level, ET = 26.9 kcal mol-1 and the molar absorption coefficient is epsilon T = 26,650 M-1 cm-1 at 720 nm. A dramatic effect of oxygen on the fluorescence (phi f) and intersystem crossing (phi T) quantum yields has been observed. The BPD-MA presents rather high triplet (phi T = 0.68 under N2-saturated conditions) and singlet oxygen (phi delta = 0.78) quantum yields. On the other hand, the presence of oxygen does not significantly modify the photobleaching of this photostable compound, the photodegradation quantum yield (phi Pb) of which was found to be on the order of 5 x 10(-5) in organic solvents.