Basic data on the physical and chemical properties measured over the period 21 August through 31 August 1978 on the tidally influenced Potomac River and estuary are presented herein. Forty-two vertical profiles, and approximately 170 surface observations were made of the distributions of salinity, temperature, light transmission, chlorophyll-a fluorescence, dissolved oxygen, partial pressure of CO2, orthophosphate, nitrate+nitrite, nitrite, ammonia, and silicate. Additional analyses were performed on discrete samples collected for particulate organic carbon, alkalinity, pH, suspended particulate matter, total phosphate, total nitrogen, total dissolved phosphate, and total dissolved nitrogen.