Dissolved organic matter (DOM) quantity and composition control the rate of formation (Rf,T) of triplet excited states of dissolved natural organic matter (3DOM∗) and the efficiency of 3DOM∗ formation (the apparent quantum yield, AQYT). Here, the reactivity of 3DOM∗ in stormflow samples collected from watersheds with variable land covers is examined. Stormflow DOM reflects variability in DOM quantity and composition as a function of land cover and may be important in controlling the fate of cotransported pollutants. Rf,T and AQYT were measured using 2,4,6-trimethylphenol in stormflow samples under simulated sunlight. The DOM source and composition was characterized using absorbance and fluorescence spectroscopies and high-resolution mass spectrometry. Rf,T and the total rate of light absorption by the water samples (Ra) increased with the dissolved organic carbon (DOC) concentration. AQYT was independent of DOC concentration, but varied with DOM source: developed land cover (4-6%) ≈ open water > vegetated land cover (3%). AQYT was positively related to an index for microbial/algal DOM content and negatively related to DOM molecular weight, DOM aromaticity, and the content of polyphenols. This work demonstrates that TMP is an effective probe for the determination of Rf,T and AQYT in whole water samples after accounting for the inhibition of TMP photodegradation by DOM.