Singlet oxygen( 1O 2)is a reactive oxygen species produced by dissolved organic matter(DOM)in sunlit waters. While the production of 1O 2 by DOM has been studied, little is known on interactions between 1O 2 and DOM. The central objective of this work was to quantify the rate constants of reaction and quenching of 1O 2 with Suwannee River and Pony Lake fulvic acids, the terrestrial and microbial end-member reference aquatic humic substances of the International Humic Substance Society. Fulvic acids were reacted with 1O 2 generated through visible light irradiation of Rose Bengal. Uptake of 1O 2 by the fulvic acids was followed through changes in dissolved oxygen concentrations via membrane inlet mass spectrometry(MIMS). Results from multiple diagnostic tests for 1O 2 - processes in solution suggested that 64-70% of the observed uptake of oxygen by the fulvic acid solutions was due to reaction with 1O 2; the remaining O 2 uptake was likely due to non - 1O 2 processes initiated by the excited-state sensitizer. The rate constants of reaction(k rxn)and physical quenching(k phys)with 1O 2 were determined to be 2.6 × 10 5 M-C -1 s -1 and 2.7 × 10 5 M-C 1- s -1(k rxn)and 1.5 × 10 5 M-C 1- s -1 and 1.3 × 10 6 M-C 1- s -1(k phys)for Suwannee River and Pony Lake fulvic acids, respectively. Results from this study demonstrated that 10 2 reacts with microbially and terrestrially derived DOM at rate constants comparable to phenols, naphthols, or aromatic amines, on a per carbon basis.