The salt N-ethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 12TFSI) undergoes several solid-solid phase transitions before melting at 91°C. At -120°C, the crystal structure consists of ordered ions (phase IV). After the solid-solid phase transition at -85°C, however, all of the PYR 12 + and TFSI - ions become disordered, forming a plastic crystalline phase (phase III). We report here the PYR 12TFSI phase III and IV structures determined from single-crystal diffraction data using the same crystal. The two structures are compared to elucidate the mechanism of the phase transition and gain a greater understanding of disordering modes of ions in the solid state.