Batch microcosms utilizing indigenous microorganisms were used to examine the effect of soil pH on the biodegradation of benzene, toluene, ethylbenzene, and m-xylene in subsurface soils. The biodegradation potential of both denitrifying and aerobic respiring microorganisms were assessed. Uncontaminated soil from five different soil depths was utilized. The pH of each soil was adjusted, such that a range of soil pH values existed at each soil depth. In this way, degradation rates of each of the aromatic compounds were determined at different soil pH values. Soil from each depth was characterized on the basis of: (1) soil particle size, (2) natural soil pH, (3) moisture content. It was suspected that the physical and chemical characteristics of a soil affected the microbial populations and the subsequent biodegradation potential of that soil. At each soil depth, microcosms under aerobic conditions experienced a higher cumulative degradation rate of the BTEX compounds than did microcosms under denitrifying conditions. The results from this study suggest that the natural soil pH did not provide an optimum environment for those aerobic respiring microorganisms which degrade BTEX. But rather, each depth in the soil profile had an optimum soil pH existing somewhere within the pH 5.4 to 6.6 range. Thus, the maximum combined degradation rate of BTEX was not found in soils at the natural soil pH. The 15 ft soil experienced the highest maximum combined degradation rate of BTEX. This soil had the highest moisture content, and relatively high sand and silt contents, which may have provided optimum conditions for microbial growth and subsequent biodegradation. / Master of Science