A detailed and reliable bioassay to detect peroxidase activity in order to select microbial catalytic agents capable to oxidize isosafrol, 4-(1'-propene)-1,2-methylenedioxybenzene, into piperonal, 4-carboxaldehyde-1,2-methylenedioxibenzene is fully described. The systematic methodology was divided into two parts: first peroxidase activity in microbial systems was detected and determined via the guaiacol assay method and secondly, a biotransformation process to oxidize the substrate was developed. Investigation assays to detect peroxidase activities were performed by using UV-Vis spectrometry, while product formation was monitored by gas chromatography. The biooxidative peroxidase catalyzed transformation of isosafrol into piperonal was achieved with 20% conversion. (C) 2003 Elsevier Ltd. All rights reserved.