The interaction of 4,5’,8-trimethylpsoralen (TMP) with molecular oxygen in a variety of solvents has been investigated using pulsed, time-resolved photoacoustic calorimetry and fluorescence emission. In this work we propose a methodology for the absolute determination of the quantum yield for the photosensitized production of singlet molecular oxygen, based on data from time resolved photoacoustics. The methodology also allows the determination of the quenching rate constant of the sensitizer triplet state by oxygen as well as the intersystem crossing quantum yield, provided the energy level of the triplet state is known. The methodology has been checked with two well-known photosensitizing agents, rose bengal and methylene blue. We found that in all of the solvents investigated, the production of singlet oxygen by TMP is a process with relatively high probability and may therefore play an important role in the overall reactivity of the molecule. For comparison, photoacoustic data for 8-methoxypsoralen (8-MOP) are reported, showing that singlet oxygen photosensitization is low in all of the solvents examined.