In aquatic systems, DOC is composed of refractory and labile compounds which are consumed by bacteria and other heterotrophic organisms. The cellular material produced through DOC consumption enters the microbial food web and may contribute significantly to fish or other yields of the system. DOC enters the water column as exudates from primary producers, as excretions from grazers, and as partially decomposed organic material leached from soils, sediments, and marshes. We investigated the distribution of DOC in Chesapeake Bay to examine the relative importance of these different sources within the estuary. DOC concentrations were measured at 10 stations on 7 cruises using the persulfate oxidation method. Property-property plots were used to compare the DOC concentrations with salinity, a conservative property, and chlorophyll a, a non-conservative property correlated with phytoplankton biomass. DOC concentrations declined from 200 μm at the freshwater end-member to 100 μm at the saltwater end-member. During May through July, DOC concentrations in the mesohaline region of the bay exceeded levels predicted by conservative mixing by 80-170 μm. From August through November, however, DOC exhibited conservative mixing. DOC was positively, but weakly correlated with chlorophyll a levels on two cruises, suggesting phytoplankton as an internal source of DOC.