Biodegradation of tetrachloroethylene (PCE) to trichloroethylene (TCE) and cis-1,2-dichloroethylene (cis-DCE) under the anaerobic condition was studied using static soil microcosms. It was proved that the biodegradation was well approximated by a consecutive firstorder reaction model. This implied that the total of the molar concentration of the above three compounds was always constant and equaled to the initial molarity of PCE, even if the amount of each compound changed as a function of time. On the basis of these facts, two indices for the characterization of the contamination were derived : total contamination level determined as the sum of the molarities of the three compounds and degradation level defined by the total molar percentage of the products (TCE+DCE). These indices enable us to evaluate the initial concentration of the pollutant PCE and the degree of degradation.