Biotransformation of the isoflavones, 6,7,4'-trimethoxyisoflavone (1) and S,7,4'-trimethoxyisoflavone (2) by Aspergillus niger was investigated. Compound 1 was transformed to 4'-hydroxy-6,7-dimethoxyisoflavone (3) and 2 to 4'-hydroxy-5,7-dimethoxyisoflavone (4). This suggested that 1 and 2 were demethylated at the C-4' position with regioselectivity by Aspergillus niger. (c) 2006 Society of Chemical Industry.