The biodegradation of the monoazo-sulphonic dye Acid Orange 7 by means of Pseudomonas sp. OX1 was investigated. Conversion was carried out in both gas-sparged flasks and in internal loop airlift reactors under controlled operating conditions at dye concentration up to 0.8 g/L. Luria-Bertani or synthetic culture media were used. Bioremediation was accomplished by a sequence of aerobic and anaerobic phases. Cell growth took place only during the aerobic phase, while dye conversion occurred only during the anaerobic phase, according to a Michaelis-Menten-type kinetics. The maximum extent of decolorization was closely correlated with the amount of substrate carbon converted during the preceding aerobic phase; a limiting conversion of 70 mg(dye) per gram of substrate carbon was measured whatever the carbon source. The study was successfully pursued further to explore the potential of a decolorization process based on cyclic aerobic-anaerobic operation of an airlift bioreactor. Whatever the initial dye concentration, dye depletion was achieved provided that optimal cyclic aerobic-anaerobic operating conditions were selected. (C) 2007 Elsevier Inc. All rights reserved.