|Groundwater is an important water resource for many communities and municipalities in our country. Due to past practices in landfill and in toxic chemical disposal, many groundwater supplies are threatened by toxic chemical contaminants. In general, most of the chlorinated chemicals are toxic as forms of aliphatic halogenated organic compounds. In this study, the optimal conditions to detoxify chlorinated compounds by the reductive degradation were investigated. The following results were obtained in the reductive degradation of CCI₄, C₂CI_(6), and C₂HCI_(5), by using FeS and FeS₂ as mediators. CCI₄ was reduced to CH₂CI₃ and CH₂Cl₂ in anaerobic conditions when FeS was used as a mediator. While the reduction of CCI₄ to CHCI₃ was rapidly proceeded, the reduction of CHCI₃ to CH₂Cl₂ was occurred slowly. Further reduction to CH₃CI was not observed. Unlike CCI₄, C₂CI_(6) was degraded to C₂HCI_(5), C₂CI₄. C₂HCI₃ and cis-1,2-C₂H₂Cl₂ by complicated pathways such as hydrogenolysis, dehalo-elimination and dehydrohalogenation. A small amount of C₂HCI_(5) was detected only in the early stages of the reduction. However, majority of the C₂CI_(6) was reduced to C₂CI₄. cis-1,2-C₂H₂Cl₂ was the only product among other possible isomers. Kinetic characteristics dependent on several factors such as iron mineral and organic solvents were investigated. When Fe^(0), FeS and FeS₂ were used as mediators, minerals affecting reaction rate were in the following order : Fe^(0) ＞ FeS ＞ FeS₂ when in contact with C₂CI_(6).