An investigation of the nutrient cycling, particularly denitrification, of a tidal freshwater marsh was conducted using flux core experiments. Sediment cores were collected from the Jug Bay Wetlands Sanctuary on the Patuxent River, MD, and incubated in a controlled environment chamber. A recently developed Membrane Inlet Mass Spectrometer (MIMS) technique was used to measure denitrification rates in the sediment core flux chambers. Denitrification rates ranged from 45 µmol N m-2hr-1 to 165 µmol N m-2hr-1 in early summer and from 27 µmol N m-2hr-1 to 160 µmol N m-2hr-1 during late summer. Although large spatial and temporal variation was seen, there was strong evidence for the importance of coupled nitrification and denitrification, as decreases in the concentration of nitrate accounted for 35-90 % of the total denitrification in early summer, and 1-8 % during late summer. Methanogenesis rates in the marsh were also substantial, and methane ebullition represents a possible vector for the removal of nitrogen during the tidal cycle. These summer denitrification rates, which were comparable to spring rates in bottom sediments of the Patuxent River, demonstrate that the marsh represents an important sink for combined nitrogen in the ecosystem.