Decolorization of phenolic compounds contained in fresh and stored-black olive mill wastewaters (OMW) by Geotrichum candidum in an aerated batch bioreactor was investigated. During storage of OMW, autooxidation and subsequent polymerization of phenolic compounds and tannins, gives rise to darkly coloured phenolic compounds which are not readily biodegradable. G. candidum growth on fresh OMW decreased pH and reduced COD by 50% removal during the first 3 days and subsequently by a further 15%. In contrast, 75% of the colour was removed during the last 3 days of culture because G. candidum hydrolyzed phenolic compounds with high molecular weight and removed many simple phenolic compounds. G. candidum growth on the stored-black OMW was rapidly inhibited resulting in low reduction COD (25%) with no decolorization because phenol polymerization was amplified by the increased pH and oxygen. The addition of oxygen to enable G. candidum growth and biodegradation of phenolic compounds is critical in order to avoid the polymerization of phenolic compounds and tannins. (C) 2002 Elsevier Science Ltd. All rights reserved.