Procyanidins commonly known as condensed tannins are a type of polyphenol with wide abundance naturally. They are commonly known as potent anti-oxidants with powerful free radical scavenging activity as well as anti-tumor-promoting activity. Little is known about the enzymatic mechanisms/pathways involved in the microbial biotransformation of these polyphenolic molecules. The extracellular enzyme, dioxygenase produced by Aspergillus fumigatus was used as in vitro tools to study the degradation pathway of a model procyanidin dimer, namely procyanidin B2. The enzyme was purified to homogeneity by a two step process of anion-exchange chromatography coupled with FPLC followed by gel-filtration chromatography coupled with HPLC and the molecular mass estimated. In addition, the different biotransformed products resulted from the dioxygenase action on P82 were purified using Reversed-Phase-High Performance Liquid Chromatography prior to their identification and characterization by structural elucidation using Electrospray Ionization-Mass Spectrometry. Subsequently, the mechanism of dioxygenase action on procyanidin dimer was defined. (C) 2010 Elsevier Ltd. All rights reserved.