Iodine oxide particles (IOPs) formed from O₃-initiated photooxidation of diiodomethane have been investigated based on the combination of a thermal desorption/tunable vacuum ultraviolet time-of-flight photoionization aerosol mass spectrometer (TD-VUV-TOF-PIAMS) with a flow reactor for the first time. Characterization of the home-made flow reactor was performed, which indicates the applicability of its combination with TD-VUV-TOF-PIAMS. Based on that, aerosol mass spectra of IOP formation from photooxidation of CH₂I₂/O₃ were studied on-line taking full advantage of both the virtues of the flow reactor and TD-VUV-TOF-PIAMS. The main chemical components of IOPs, including atomic and molecular iodine (I, I₂), iodine oxides (IO, OIO, I₂O and I₂O₃) and hydrogen-containing iodine species (HI, HIO and HIO₃), were observed and identified based on the corresponding photoionization energy (PIE) curves, and the probable chemical composition and formation mechanism of IOPs were proposed. The work has not only improved the understanding of the formation mechanism of IOPs, but also demonstrated the capability of TD-VUV-TOF-PIAMS for direct molecular characterization of aerosols in flow reactor experiments, whose potential application in mass spectrometric studies of atmospheric aerosols is anticipated.