Measurement on the retention of cigarette smoke components in the human mouth is important for evaluating smoking-related diseases.Here,a newly developed on-line vacuum ultraviolet photoionization time-of-flight mass spectrometry(VUV-PI-TOFMS)method was applied for the mouth retention study of gaseous mainstream cigarette smoke components.The cigarette smoke in the smoking machine or in the mouths of subjects was directly introduced into the vacuum chamber,photoionized and analyzed by a TOF mass spectrometer without any tedious collection and pretreatment procedures.Thus,the retained amount of cigarette smoke components in the mouth could be observed in real-time.The results show that at 2 second holding time,aldehydes,ketones and amine compounds have high mouth retention; methanthiol,phenol and nitrogen heterocyclic compounds have medium mouth retention,while the retention of NO,benzene and unsaturated hydrocarbons were relatively low.Moreover,the effects of the holding time on the mouth retention of the gaseous components within 26 seconds were also obtained,and two main variation trends were observed for different smoke constituents.This method can offer new analysis techniques for further studying the whole respiratory tract retention in cigarette smoke,and deeply evaluating the relationship between smoking and health.