The optimization of operating conditions for glucose oxidation to gluconic acid was systemically studied using an ultra-scaled down membrane bioreactor based on the parameter scanning ultrafiltration apparatus. The bioconversion of glucose to gluconic acid was carried out with freely suspended glucose oxidase (EC 1.1.3.4) as catalyst and dissolved oxygen saturated with air as oxygen supply. The bioreactor used PES ultrafiltration membrane with a MWCO of 70 kDa, which proved to retain glucose oxidase completely. The effects of pH, temperature, salt concentration, present of catalase, substrate-to-enzyme ratio, permeate flux and stirring speed on glucose conversion were examined. The results show that the addition of sodium chloride or catalase had a negative effect on glucose conversion. Under the most suitable conditions, a conversion of about 53.8% could be attained using the experimental apparatus. The identified operational and physicochemical conditions can guide the design and operation of enzymatic membrane bioreactor for partial oxidation of glucose to gluconic acid. (c) 2007 Elsevier B.V. All rights reserved.