Singlet oxygen plays a major role in photodynamic inactivation of tumor cells or bacteria. Its efficacy depends critically on the oxygen concentration [O(2)], which can decrease in case oxygen is consumed caused by oxidative reactions. When detecting singlet oxygen directly by its luminescence at 1270 nm, the course of the luminescence signal is critically affected by [O(2)]. Thus, it should be feasible to monitor oxygen consumption during photo-oxidative processes. Singlet oxygen was generated by exciting a photosensitizer (TMPyP) in aqueous solution (H(2)O or D(2)O) of albumin. Chromatography shows that most of the TMPyP molecules are unbound, and therefore singlet oxygen molecules can diffuse in the solution. A sensor device for oxygen concentration revealed a rapid decrease of [O(2)] (oxygen depletion) in the solution during irradiation. The extent of oxygen depletion in aqueous albumin solution depends on the radiant exposure and the solvent. When detecting the luminescence signal of singlet oxygen, the shape of the luminescence signal significantly changed with irradiation time. Thus, local oxygen consumption could be monitored during photodynamic action by evaluating the course of singlet oxygen luminescence.