Performance of single- and two-stage pH-stat completely stirred tank reactor (CSTR) systems for anaerobic acrylic acid biotransformation was investigated and compared. In the experiments, acetate was added with acrylic acid. Acetate concentration in both single- and two-stage reactors was continuously maintained at a nonlimiting concentration of 2000 +/- 200 mg/L. using a computer-controlled pH-stat system to allow observation of the effect of acrylate biotransformation on the more sensitive methanogenic step. For this purpose, conventional (one-stage) and two-stage pi-I-stat CSTR systems with the same acrylic acid loading strategy were operated under the same conditions, and their performances were compared. The food-to-microorganism ratio in the first stage of the two-stage system was maintained at approximately two times that of the one-stage system. Furthermore. relatively high acrylate concentrations and dilution rates were achieved in the first stage of the two-stage system. Results revealed that the two-stage pH-stat CSTR system provided better effluent quality and more stable operation with the toxic effects of acrylic acid and its intermediate products than the single-stage system.