A chemical model system for cytochrome P-450, consisting of tetraphenylporphyrin manganese chloride (TPPMnCl) and iodosylbenzene, efficiently oxidized 2-methylimidazole to 2-methylimidazolone. This system was next applied to 4-(2-methyl-1-imidazolyl)-2,2-diphenylbutyramide, a muscarinic acetylcholine receptor antagonist under clinical trial, affording the previously unisolated imidazole ring 5-mono-oxidized derivative that is considered to be the precursor of the main metabolites. This system, which is superior to the copper-ascorbate system, should be applicable to in vitro studies of various drugs containing the 2-methylimidazole moiety.