Abstract: Cigarette mainstream smoke is a complex aerosol system that consists of gas-phase and particulate-phase components. The chemical compositions of both phases continuously change with time and temperature, and can significantly impact human health in various ways. Many components in both phases are toxic or carcinogenic, and they are dynamically partitioned between the two phases. Current research on the chemical composition of cigarette smoke aerosols mainly focuses on off-line detection, particularly the particulate-phase, which presents a significant challenge for on-line detection. In order to accurately characterize the chemical composition of the two phases, a homemade vacuum ultraviolet photoionization mass spectrometer was utilized to analyze cigarette mainstream smoke. The high time resolution of the instrument was allowed for thorough investigation of the puff-by-puff characteristics of cigarette smoke. By combining a capillary interface and an aerodynamic lens to sampling, the instrument precisely analyzed the components of gas-phase and particulate-phase, respectively. The experimental results demonstrated that the gas-phase and particulate-phases of cigarette mainstream smoke have their own unique and distinctive mass spectra. The gas-phase components mainly consisted of small molecular volatile organic compounds with mass numbers in the range of m/z <150, while the particulate-phase components were more intricate and could cover the entire mass range detected by the mass spectrometer (m/z <400). Some components primarily presented in the gas phase, some existed solely in the particulate phase, while others could be found in both phases. The puff-by-puff analysis of the gas phase showed two variations, the signal intensity of most components tended to increase with the puff number. However, some unsaturated hydrocarbons exhibited higher relative intensity in the second puff, after which it gradually decreased and eventually stabilized or slightly increased. Almost all the components in the particulate-phase showed an increasing puff-by-puff trend, which was gentler compared to that of the gas-phase components. This study resolves the chemical composition of fresh cigarette smoke on-line and characterizes the partitioning of some toxic or carcinogenic compounds in the gas-phase and particulate-phase, which is important for understanding the health effects of mainstream cigarette smoke, and also provides detailed information on the dynamics of important components in both phases during smoking, which is expected to facilitate quantitative or semi-quantitative investigation of the component partitioning in the gas-phase and particulate-phase in the future.