Biodegradation of petroleum hydrocarbons (PHC) and 16 polycyclic aromatic hydrocarbon (PAH) compounds was evaluated in sediments from a stormwater basin that was a former salt marsh at an active oil refinery. Spartina patens were grown in basin sediments during a field study and a greenhouse study over three consecutive growing seasons. Ninety percent (90%) survival was observed in field study plots containing sediments with <12% PHC. In the greenhouse study, planted and bulk sediments were monitored for PHC, PAH, microbial density, nutrients, pH, redox and root biomass at 2.5 and 7.5 cm in sediments with <12% PHC. The sediments contained on average 6.5% to 9.5% PHC and 56 to 124 parts per million (ppm) total PAH. The plants exhibited a 95% survival rate and added from 1% to 2% root biomass to the sediment. Microbial densities in planted sediments were significantly higher than in bulk sediments (p<0.05). Oxidizing conditions were prevalent in all treatments at both depths; however, reducing conditions developed in the planted sediments at 7.5 cm. PHC was 35% to 37% lower in the planted sediments vs. bulk sediment at the end of the study. Planted sediment PAH concentrations were generally lower than in bulk sediments. Low MW PAHs (2 and 3-Rings) were either absent or at relatively low concentrations in all treatments. Phenanthrene concentrations in bulk sediments did not change, but were reduced by 95% in planted sediments (p<0.01), with half-life estimates of 141 to 165 days at 2.5 cm. Temporal changes in high MW PAHs ( 4-Rings) were not observed over the greenhouse study period. Differences in PAH concentrations between planted and bulk sediments exhibited a decreasing trend with PAH molecular weight. Evaluation of differences in planted and bulk sediment PAH concentrations suggests that lower MW PAHs degrade better under oxidizing conditions, and higher MW PAHs under reducing conditions in the planted sediments. Comparison of planted and bulk sediment PAH concentrations links depth and redox conditions to decreases in PAH as a function of PAH MW. Therefore, it was concluded that spatio-temporal variations and cycling of redox conditions should be considered to properly evaluate biodegradation of PAH.