A multiyear field study was undertaken on Lake Superior to investigate singlet oxygen (1O2) photoproduction. Specifically, trends within the lake were examined, along with an assessment of whether correlations existed between chromophoric dissolved organic matter (CDOM) characteristics and 1O2 production rates and quantum yields. Quantum yield values were determined and used to estimate noontime surface 1O 2 steady-state concentrations ([1O2] ss). Samples were subdivided into three categories based on their absorbance properties (a300): riverine, river-impacted, or open lake sites. Using calculated surface [1O2]ss, photochemical half-lives under continuous summer sunlight were calculated for cimetidine, a pharmaceutical whose reaction with 1O2 has been established, to be on the order of hours, days, and a week for the riverine, river-impacted, and open lake waters, respectively. Of the CDOM properties investigated, it was found that dissolved organic carbon (DOC) and a300 were the best parameters for predicting production rates of [1O 2]ss. For example, given the correlations found, one could predict [1O2]ss within a factor of 4 using a300 alone. Changes in the quantum efficiency of 1O2 production upon dilution of river water samples with lake water samples demonstrated that the CDOM found in the open lake is not simply diluted riverine organic matter. The open lake pool was characterized by low absorption coefficient, low fluorescence, and low DOC, but more highly efficient 1O2 production and predominates the Lake Superior system spatially. This study establishes that parameters that reflect the quantity of CDOM (e.g., a300 and DOC) correlate with 1O2 production rates, while parameters that characterize the absorbance spectrum (e.g., spectral slope coefficient and E2:E3) correlate with 1O2 production quantum yields.