Computer controlled reactors were used to determine the maximal rate of anaerobic biodegradation of chloroform and trichloroethylene using three important anaerobic intermediates (propionate, hydrogen, and acetate) as primary substrates. Maximal biodegradation rate was defined as that loading rate of chloroform and trichloroethylene which can be achieved while reducing process efficiency of the primary substrate to 50 %. The systems were controlled by a computer in response to the pH of the reactor in order to establish the unlimited equilibrium utilization levels of the three primary substrates and the chlorinated aliphatic compounds. From 89 to 99 % of chloroform and trichloroethylene was biodegraded at maximal loading rate of 15-109 mg/l of reactor-day in the primary substrate enrichment cultures. Biodegradation potentials, the affected class of microorganisms, and the fate and metabolic intermediates of chloroform and trichloroethylene also were examined.