Results are presented and discussed from a preliminary study on the oxygen isotopic composition of coastal and inner shelf waters from the Middle Atlantic Bight between the mouths of Delaware Bay and Chesapeake Bay. The water samples were collected during the spring, April/May 1993, in conjunction with a detailed hydrographic survey of the Delaware Coastal Current. The relationship between salinity and δ18O, both of which are generally accepted conservative properties of seawater, allows identification of the fresh and saline mixing end-members for the Delaware Coastal Current. The main source of freshwater to the Delaware Coastal Current is a mixture of runoff into Delaware Bay with an estimated δ18OS = 0 composition of −7.08‰. The freshwater input is dominated by Delaware River discharge with a mean δ18O of −8.0‰ based on previous studies; there appears to be a secondary contribution of freshwater that is isotopically heavier from coastal-plain drainage. The extrapolated δ18OS = 32.35 value (−0.93‰) of the saline component implies exchange with Middle Atlantic Bight Shelf Water through along-shelf and across-shelf mixing processes. Limited data suggest that the oxygen isotopic composition of low-salinity water exiting the Chesapeake Bay mouth is discernible compared with that of the Delaware Coastal Current.