Denitrification measurements were made under simulated field conditions using sediment cores and water collected from the Hemet/San Jacinto Multipurpose Demonstration Wetland (Riverside County, California, USA). The 9.9 ha constructed wetland is used to both polish ammonia-dominated secondary municipal effluent and provide migratory bird habitat. The wetland was originally constructed as a marsh-pond-marsh system in 1994. Over the period from January through March 1998, measured denitrification rates averaged 20.9 ± 20.9 μmol N m⁻² h⁻¹ within the emergent marsh portions of the wetland and 646 ± 353 μmol N m⁻² h⁻¹ in open water areas. The mean areal denitrification removal rate for this period was 0.70 kg N ha⁻¹ d⁻¹, which accounted for about 8% of the total N removed by the wetland. Internal retention was the main N-removal mechanism. Synoptic water quality surveys indicated that denitrification was limited by a lack of nitrification within the wetland. Between April 1998 and January 1999, the wetland was reconfigured as a hemi-marsh system, having equal areas of interspersed emergent marsh and deep open water. In May 1999, measured denitrification rates averaged 1414 ± 298 μmol N m⁻² h⁻¹ within the emergent marsh zones and 682 ± 218 μmol N m⁻² h⁻¹ in the open water areas. The mean areal denitrification removal rate was 3.58 kg N ha⁻¹ d⁻¹, which accounted for 40% of the total N removed by the wetland. A synoptic water quality survey indicated that nitrification within the wetland had been enhanced by the reduction of emergent macrophyte biomass and the increase in the area of interspersed deep open water. The modifications to the wetland shifted the nitrogen balance from a large internal storage component and a small denitrification component in 1998 to a more denitrifying system in 1999.