A two-phase partitioning bioreactor containing Pseudomonas putida ATCC 11172 was used to degrade high concentrations of phenol in batch and fed-batch mode. The 2-1 (nominal volume) partitioning bioreactor employs a 1-1 cell-containing aqueous phase, and a 500-ml immiscible and biocompatible second organic phase (2-undecanone), which partitions the toxic substrate into the aqueous phase at a rate based on the metabolic activity of the microorganisms. Using this reactor configuration, operated in batch mode, 10-g phenol was degraded to completion within 84-h, The system was, however, oxygen-limited during the rapid growth phase of the fermentation. A second experiment, using enriched air to prevent oxygen limitation, resulted in the complete degradation of 10-g phenol within 72-h. The use of a sequential feeding strategy, in which a IO-g phenol load was added in sequential 5-g aliquots, resulted in a significant reduction in the lag phase? from 36-h to 12-h, and the consumption of 10-g phenol in 60 h. Finally, fed-batch fermentation was used to attempt to determine the ultimate capacity of the system to degrade phenol. The organic phase was loaded with 10-g phenol, the microorganisms were allowed to consume this aliquot almost to completion, and a second 10-g, aliquot was then added. The organic phase was spiked in this manner a total of four times, resulting in the degradation of 46.55-g phenol within 12 days. The system was also monitored for nutrient depletion, and a nutrient-feeding schedule was formulated, in response to the mass of phenol consumed.