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This study presents the method of fixed-photon-energy double-imaging photoelectron photoion coincidence (i²PEPICO) utilized to investigate the dissociation of state-selected ions. Vacuum ultraviolet (VUV) synchrotron radiation at one fixed photon energy of hν = 21.2 eV, the He(I) atomic resonance energy, is employed as a light source to ionize molecules. Various dynamic information including time-of-flight (TOF) mass spectra, mass-selected photoelectron spectra (PES), and electron and ion kinetic energy correlation diagrams corresponding to each mass are obtained efficiently with the multiplexed capabilities of i²PEPICO, thereby revealing the detailed dissociation mechanisms of ions. As representative examples, dissociation of state-selected O₂⁺ ions prepared in the b⁴∑_g⁻ and B²∑_g⁻ electronic states and CH₃F⁺ ions in the X²E, A²A₁, and B²E states were selected and investigated.