A microcosm study was performed to investigate the biodegradation potential of BTX compounds in unsaturated soils under aerobic and anaerobic conditions. Toluene was used as a model compound at concentrations of 100 to 200 mg/L. An uncontaminated, groseclose soil ranging in depth from 0 to 18 feet was used in order to observe differences in microbial degradation abilities in shallow subsurface environments. Several metabolic groups were investigated including aerobes, denitrifyers and sulfate reducers. Bacterial densities of these groups were determined at each soil depth. Physical and chemical parameters of the subsurface environments were also investigated to observe their impact on microbial biodegradation potentials. These included changes in soil particle size, moisture content, and pH with increasing depth. Substantial toluene biodegradation took place in some, but not all soils under both aerobic and denitrifying conditions. Biodegradation rates varied considerably among aerobes and denitrifyers in similar environments. In acidic, poorly drained clay soils of 3, 6 and 9 feet, denitrifyers readily degraded toluene while aerobic microorganisms were unable to mineralize the compcund. Evidence of toluene biodegradation by anaerobic bacteria including sulfate reducers was also observed in moist, clay soils although the rates were much slower. Currently, in situ bioremediation techniques for benzene, toluene and xylene compounds almost always rely on indigenous, aerobic organisms to degrade the contaminants. However, results of this study indicate that aerobic organisms capable of biodegradation may not exist in some subsurface environments, even in soils very close to the surface. The physical/chemical properties of unsaturated environments have significant impacts on microbial capabilities as well as the biodegradation potential of contaminants. / Master of Science