During toluene hydroxylation, catalyzed by Pseudomonas putida UV4 one molecule of oxygen is added to the aromatic ring to produce the dihydroxylated (non-aromatic) ring structure, toluene cis-glycol. Toluene, which is toxic to the cells at aqueous phase concentration above (similar to2.4 mmol), is fed to the reactor. A feed-back control system based on oxygen uptake rate measurements was used to control the feed rate, and thus maintain the aqueous phase toluene concentration in the desired range for zero order kinetics. (C) 2001 Elsevier Science Inc. All rights reserved.