Leonardite is a naturally occurring oxidized form of lignite coal that is rich in humic materials. It is collected during coal-mining procedures, as it is commonly found overlying coal seams. Humic substances have been shown to significantly enhance the aqueous solubilities of PAHs by acting as a surfactant, increasing the water solubility of the PAH and allowing the contaminants to enter the aqueous phase. The use of humic acids on contaminated soils has the potential to greatly enhance the solubility, and consequently the bioavailability, of PAHs allowing for an easier and more efficient bioremediation procedure. A fluorescence quenching method was used to determine the association constants (Kb) for various water-soluble humic materials and naphthalene. The objective of the microcosm study was to determine if a variety of dissolved humic material solution amendments could influence the biodegradation rate of selected PAHs, namely anthracene and benzo[a]pyrene, in a soil with previous exposure to PAHs. (Abstract shortened by UMI.)