Abstract— The hematoporphyrin-sensitized production of singlet molecular oxygen, O2(1Δg), has been investigated in methanol and in aqueous solution. The quantum yield for formation of O2(1Δg) (ΦΔ) has been measured by both steady-state (oxygen consumption) and time-resolved (near-infrared luminescence) methods. In methanol, both techniques indicate that ΦΔ= 0.76 and the value remains independent of sensitizer concentration over a wide range. This finding is consistent with the dye persisting in a monomelic form in methanol solution. In contrast, ΦΔ decreases markedly with increasing sensitizer concentration in water due to dimerization of the dye. Analysis of the steady-state data indicates ΦΔ values of 0.74 and 0.12, respectively, for monomer and dimer. It is further shown that the efficiency whereby quenching of the triplet state by O2 results in generation of O2(1Δg) is substantially lower for the dimer than for the corresponding monomer. Because monomer and dimer possess quite different absorption spectral profiles, the efficacy for photodynamic action with hematoporphyrin exhibits a pronounced wavelength dependence.