Smoking is a main cause of premature death and preventable disease in the world. Interestingly, animal studies indicate that inhibition of monoamine oxidase (MAO), key enzymes for the degradation of neurotransmitters, increased self-administration of nicotine. The purpose of this study was to identify and characterize the potential MAO inhibitors in tobacco smoke responsible for MAO inhibition in smokers. A bioassay-guided isolation from an extract of tobacco smoke showed that catechol, 4-methylcatechol, hydroquinone, α-linolenic acid, and linoleic acid all displayed potent human MAO inhibitory activity. Additionally, the tobacco catechols 4-ethylcatechol and 4-vinylcatechol were included to test their inhibitory potencies. Catechol, 4-methylcatechol, 4-ethylcatechol, and hydroquinone are potent and irreversible MAO inhibitors. Among the phenolic compounds tested, 4-methylcatechol and 4-ethylcatechol inhibited MAO A with IC