To detect singlet oxygen ((1)O2), the commercially available fluorescent sensor named Singlet Oxygen Sensor Green (SOSG) has been the most widely used from material studies to medical applications, for example, photodynamic therapy. In light of the previous studies, SOSG is a dyad composed of fluorescein and anthracene moieties. In the present study, we carried out quantitative studies on photochemical dynamics of SOSG for the first time, such as the occurrence of intramolecular photoinduced electron transfer (PET), (1)O2 generation, and two-photon ionization. It was revealed that these relaxation pathways strongly depend on the irradiation conditions. The visible-light excitation (ex. 532 nm) of SOSG induced intramolecular PET as a major deactivation process (kPET = 9.7 × 10(11) s(-1)), resulting in fluorescence quenching. In addition, intersystem crossing occurred as a minor deactivation process that gave rise to (1)O2 generation via the bimolecular triplet-triplet energy transfer (kq = 1.2 × 10(9) M(-1) s(-1)). Meanwhile, ultraviolet-light excitation (355 nm) of SOSG caused the two-photon ionization to give a SOSG cation (Φion = 0.003 at 24 mJ cm(-2)), leading to SOSG decomposition to the final products. Our results clearly demonstrate the problems of SOSG, such as photodecomposition and (1)O2 generation. In fact, these are not special for SOSG but common drawbacks for most of the fluorescein-based sensors.