Microbial activity regulates critical ecosystem functions. In 2010, the Deep-water Horizon oil platform exploded triggering the largest marine oil spill in US history releasing 5 million barrels of oil into the Gulf of Mexico. The dispersant Corexit use totaled 7.9 million liters during the spill. Coastal Louisiana sediment samples were collected from an unimpacted Spartina coastal marsh site near areas that had been heavily impacted by the spill. The Denitrification rate was determined for the following treatments; fresh (non-weathered) oil, weathered oil, Corexit and Control (non-impacted). Denitrification rates for weathered oil and fresh oil were 87.9±1.2% and 51.5±5% of the control, respectively. Fresh oil contained larger amounts of volatile compounds and had a greater impact on denitrification rates. Denitrification rates were also determined in the presence of Corexit at five ratios of dispersant to field moist soil weight (0:10, 1:10, 1:100. 1:1000, 1:10000). The denitrification rates for the; 1:10 ratio was 0% of the control; 1:100 was 7.6±2.7% of the control; 1:1,000 was 33±4.3% of the control; and 1:10,000 was 97±4.9% of the control. These data suggest that denitrifying bacteria are inhibited by chemical dispersants, and the use of dispersants has the potential to harm overall water quality by diminishing the capacity of the marsh environment to process and remove nitrate. Longer term studies need to be conducted to see if or when the denitrifying microbes recover from oil and dispersant additions.