2-Anthraquinone sulfonate (2-AQS) was found to exhibit an abnormally high singlet-oxygen quantum yield in aqueous solution after UVA irradiation based on a widely used testing method for singlet oxygen. To discover the cause of the abnormal results, several possible reactive oxygen species were also measured in the system. Results revealed that 2-AQS and two other similar compounds produced singlet oxygen, hydroxyl radicals, and superoxide, so-called reactive oxygen species (ROS), under UVA exposure. The production of hydroxyl radical contributed to the abnormally high quantum yield. Quantifications of the ROS produced by these compounds were conducted by oxidation of p-nitrosodimethylaninline in the presence and absence of either L-histidine or superoxide dismutase. EPR results also confirmed the formation of singlet oxygen by these compounds under UVA irradiation. The influence of different exposure times on the formation of ROS was measured for all three photosensitive compounds. The effects of different additives on the production of ROS were also investigated. The results showed that some compounds traditionally known as "hydroxyl radical scavengers" instead served as hydroxyl radical "generation promoters" under UVA exposure in the system. Moreover, for the first time, these photosensitive compounds were used to degrade a textile colorant, namely, Reactive Black 5, achieving the total decoloration of this compound within 10 min.