Contamination of groundwater resources by halogenated compounds spurred the formation of a national program to clean up hazardous waste sites across the United States. Compounds such as carbon tetrachloride (CTET), chloroform (CF), and hexachloroethane (HCA) are a few of the chemicals which have been proposed to be 'characteristic' hazardous wastes to be included in the toxic contaminant leachate potential (TCLP) test. Consequently, chemical and biological transformation pathways are being studied to aid in understanding the fate of these contaminants in groundwater environments and to apply the processes occurring naturally in groundwater environments to remediation technologies. Environmental factors significantly affect the transformation rates and the pathways of halogenated aliphatic compounds. The authors have studied the transformation of tetrachloromethane (CTET), and hexachloroethane (HCA) in homogenous and heterogenous systems designed to simulate groundwater and sediment conditions. The laboratory studies were aimed at (1) identifying the sediment components which may act as reducing components and (2) quantifying the environmental factors which govern the transformation rates. Both model and natural systems were studied. Their data indicate that the humic acid fraction in combination with sulfide and Fe(2+) may promote transformation rates. Similarly, surfaces of sheet silicates, such as biotite and vermiculite, were found to promote degradation of CTET and perchloroethylene, respectively.