Singlet oxygen (1O2) is a reactive oxygen species produced by dissolved organic matter (DOM) in sunlit waters. While the production of 1O2 by DOM has been studied, little is known on interactions between 1O2 and DOM. The central objective of this work was to quantify the rate constants of reaction and quenching of 1O2 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 1O2 generated through visible light irradiation of Rose Bengal. Uptake of 1O2 by the fulvic acids was followed through changes in dissolved oxygen concentrations via membrane inlet mass spectrometry (MIMS). Results from multiple diagnostic tests for 1O2-processes in solution suggested that 64-70% of the observed uptake of oxygen by the fulvic acid solutions was due to reaction with 1O2; the remaining O2 uptake was likely due to non-1O2 processes initiated by the excited-state sensitizer. The rate constants of reaction (krxn) and physical quenching (kphys) with 1O2 were determined to be 2.6 x 10(5) M-C(-1) s(-1) and 2.7 x 10(5) M-C(1-) s(-1) (krxn) and 1.5 x 10(5) M-C(1-) s(-1) and 1.3 x 10(6) M-C(1-) s(-1) (kphys) for Suwannee River and Pony Lake fulvic acids, respectively. Results from this study demonstrated that1O2 reacts with microbially and terrestrially derived DOM at rate constants comparable to phenols, naphthols, or aromatic amines, on a per carbon basis.