Abstract: In order to determine the human buccal residues of harmful components from cigarette smoke, an on-line measurement system based on ultraviolet photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS) technique was established. The photoionization mass spectra of fresh vapour phase mainstream cigarette smoke generated from a smoking machine directly or retained in the mouth of volunteers for different times were obtained. The retention efficiencies of the main toxic components from cigarette smoke in buccal cavity, such as nitric oxide (mass-charge ratio, m/z=30), acetaldehyde (m/z=44), 1,3-butadiene (m/z=54), acetone (m/z=58), isoprene (m/z=68), benzene (m/z=78), phenol (m/z=94) and ethylbenzene (m/z=106) were analyzed comparatively according to the results of qualitative analysis. The results showed that: 1) When cigarette smoke was retained in mouth for 2 s, the retention efficiencies of buccal cavity for aldehyde and ketone compounds were the highest, followed by phenol; while those for nitric oxide and unsaturated hydrocarbons were relatively lower. 2) Within 26 s, the longer the retention time, the higher the retention efficiencies for aldehyde and ketone compounds; and those for the other compound were also raised rapidly within the first 10-14 s, then nearly kept constant. VUV-PI-TOFMS achieved direct smoke sampling and analysis from both smoking machine and human buccal cavity, and hence is a powerful tool to study the buccal residues of harmful components from cigarette smoke.