Trimethylbenzenes (TMBs) are mono-aromatic compounds found in crude oil. These recalcitrant compounds are encountered frequently in the soil at an abandoned oil refinery site in south-central Michigan. Total TMB isomer concentrations in soil collected from a historical refined petroleum spill area range from 3,000 to 30,000 ug/kg. During the summer of 2003, an ex situ pilot study was conducted in 45 cubic yd cells to evaluate the effectiveness of treating soil contaminated with aromatic volatile compounds, including TMBs. The effectiveness of plant-assisted biodegradation using both seeded and planted prairie grasses was evaluated in an enhanced rhizodegradation test cell through chemical and biological monitoring and compared to a control (unplanted but tilled or landfarmed) cell. In 2003, soil analyses indicated a 99% TMB reduction in the test cell over a 28-week period. Confirmation of rhizosphere degradation was supported in the test cell through increased heterotrophic bacteria counts (HBC) from an average of 9.7 million colony-forming units (CFU) in the first 14 weeks to 11.8 million CFU and then up to 21 million CFU in the remaining weeks of monitoring. Increased HBC also were observed in soil collected from the rhizodegradation test cell and inoculated with various petroleum hydrocarbons, suggesting that some microbial communities do well in the presence of certain compounds when grasses are present. In June 2007, both cells were monitored to evaluate plant diversity (including presence of invasive plants), concentration of contaminants, and microbial activity. After 4 years, the predominant grass type on the test cell was Elymus (both Canadian and Virginia rye) at 52% of the vegetative cover, with 26% in planted prairie grasses, 21% in invasive pioneering species, and less than 1% bare soil. Wildlife-friendly and aesthetic species--big blue stem, little blue-stem, and side oats grama grass--had died out. The average above-grade plant mass on the test cell was 838 g/m2. The control cell showed 46% invasive species, 36% bare soil, and 18% non-specific non-planted grasses, with an average above-grade plant mass of 598 g/m2. In the soil samples collected in June 2007, TMBs were not detected in the test cell (<1 ug/kg), but were present in the control cell at a concentration of 210 ug/kg. HBC also were higher in the test cell (7.5 million CFU), compared to the control cell (0.282 million CFU). A primary challenge for full-scale implementation of an enhanced rhizodegradation remedy in former refining areas will be competition between invasive species and sensitive, less tolerant prairie species. Future site redevelopment efforts at the former refinery likely will integrate the use of prairie grasses for contaminant polishing in areas of shallow petroleum contamination and in areas designated for recreational or nature park usage. These areas will require invasive plant species control to be effective.