Abstract: In order to identify the isobars/isomers in the gas phase components of mainstream cigarette smoke, an on-line measurement system for photoionization efficiency (PIE) curve was established. The system was mainly composed of a single-port smoking machine, a three-port injector and a synchrotron radiation vacuum ultraviolet photoionization time-of-flight mass spectrometry (SR-VUV-PIMS). The PIE curves of different mass-to-charge ratio(m/z)ions in the gas phase components of mainstream cigarette smoke trapped under the ISO(International Standardization Organization)smoking regime were measured in the photon energy range of 8.0-10.6 eV. The isobars/isomers of some selected m/z were identified by the simulation method of PIE curve based on multiple linear regressions(MLR). The results showed that the device and method developed in this work could not only realize the comprehensive and accurate identification of isobars/isomers in the gas phase components of mainstream cigarette smoke, but could also determine the mole percentages of each compound corresponding to one m/z.