Singlet oxygen ((1)Delta (g)) was generated by the photoactivated photosensitizer photofrin, which is clinically approved for photodynamic therapy (PDT). Using a new infrared sensitive photomultiplier, the complete time dependence of singlet oxygen luminescence in D2O was measured. All relevant relaxation rates and rate constants for the triplet T-1 state of photofrin and for singlet oxygen were determined. For the first time, an effective back transfer of energy from singlet oxygen to the triplet state of photofrin was found, affecting the rise and decay rates of the luminescence of singlet oxygen.