Reactive intermediates form when dissolved natural organic matter (DOM) absorbs sunlight in surface waters. These reactive intermediates include triplet excited states of dissolved organic matter (T*), reactive oxygen species, carbonate radical, and halide radicals. They are associated with a variety of physicochemical processes, including carbon and metal cycling, pathogen inactivation, and reactions with trace organic contaminants. T* is particularly important in these processes because it can react either through electron or energy transfer mechanisms and it is responsible for the formation of secondary reactive intermediates, such as singlet oxygen and radicals. The quantity and composition of DOM are key variables that control the rate and efficiency of T* formation, defined as the ratio of the rate of T* formation to the total rate of light absorption. As DOM is transported through aquatic environments, its composition is altered by natural and anthropogenically-influenced biogeochemical processes. Here, DOM composition is related to the reactivity of T* in stormwater and in temperate wetlands, two important aquatic systems involved in the production and transport of DOM. The rate and efficiency of T* formation were measured with two chemical probes, 2,4,6-trimethylphenol and trans,trans-2,4-hexadienoic acid, that quantify rates of T* electron transfer and energy transfer, respectively. DOM composition was characterized using absorption spectrophotometry, fluorescence spectroscopy, and Fourier transform ion cyclotron mass spectrometry. Within our sample set, the observed range in the efficiency of T* formation is <1%–14%, and shows a distinct dependence on watershed vegetative land cover and open water extent. The rate of T* formation increases with the concentration of dissolved organic carbon (DOC) while the efficiency of T* formation is independent of DOC. The data reported here suggests that DOM derived from vascular plants has a dual role, controlling both the rate of light absorption and the efficiency of T* formation.