Abstract The main product of the horseradish peroxidase-mediated oxidative coupling of 3-methylindole by H 2 O 2 has been found to be the same compound previously obtained from chemical, radiolytic, and anodic oxidation of 3-methylindole and claimed to be a hexahydrofurodiindole of undefined stereochemistry ( meso versus d , l ). Single-crystal X-ray diffraction analysis of the compound, aimed at determining the stereochemistry, revealed that the actual product is an isomer, 7,8-dimethyl-2-oxa-4,11-diazadibenzo[ e , i ]tricyclo[5.4.0.0 3,8 ]undecane that derives from hydrolytic reorganization of the initially formed biindoline radical coupling product.