A fundamental understanding of the structure and dynamics of organic ionic plastic crystal (OIPC) materials allows for a more rational design of molecular chemistry toward improved mechanical and electrochemical performances. This Letter investigates the solid-state structure and ion dynamics of two imidazolium-based protic organic ionic plastic crystals as well as the ion-transport properties in both compounds. A combination of DSC, conductivity, NMR, and synchrotron X-ray studies revealed that a subtle change in cation chemistry results in substantial differences in the thermal phase behavior, crystalline structures, as well as the ion conduction mechanisms in the protic plastic crystal compounds. Whereas most of the research nowadays has been focused on the optimization of chemistry of cations and anions, this work highlights the importance of microstructures on the ion transport rate and pathways of the OIPC materials.