Meta(tetrahydroxyphenyl)chlorin (m-THPC) is a new photosensitizer currently undergoing clinical trials at Lausanne's CHUV hospital for photodynamic therapy (PDT) of early cancer in the upper aerodigestive tract. The illumination of m-THPC with light at 650 nn in aqueous solution containing 10% foetal calf serum (FCS) causes two simultaneously occurring processes: its photodegradation and the formation of a more stable photoproduct absorbing at 320 mn. The photodegradation quantum yield (Phi(Pb)) of m-THPC is found to be of the order of 1.5 x 10(-5) in 10% FCS. A strong dependence on oxygen concentration of the photodegradation and the formation of photoproducts has been observed. Indeed, the m-THPC presents rather low Phi(Pb) under Nz-saturated conditions: 6.9 x 10(-6). In aerobic conditions, the photodegradation as well as the formation of photoproducts, have been competitively inhibited by known singlet oxygen (O-1(2)) quenchers. The addition of superoxide dismutase (SOD), catalase or desferal, known quenching agents of type I mechanisms, has little or no effect on the rate of photobleaching and photoproduct formation of m-THPC. m-THPC generates O-1(2) With a quantum yield of 0.3 in ethanol solution as determined by photo-oxidation experiments using 1,3-diphenylisobenzofuran (DPBF) as substrate. The rate and quantum yield of DPBF photo-oxidation are found to increase with increasing substrate concentration and decrease in phosphate buffer solution (Phi(Delta) = 0.01), due to the partially hydrophilic character of m-THPC. In addition, the reaction of O-1(2) with TEMP (2,2,6,6-tetramethyl-4-piperidone) in combination with electron paramagnetic resonance (EPR) detection has been used to determine the formation of O-1(2) by m-THPC in ethanol solution. (C) 1998 Elsevier Science S.A. All rights reserved.