N,N-Dimethylpiperidinium iodide (PIP11I) has P21/n symmetry at -150 °C. The crystal packing of the ions of PIP11I is compared with that reported for N,N-dimethylpyrrolidinium iodide (PYR 11I) and N,N-dimethylmorpholinium iodide (MORP11I). Despite the differences in cation structure, PYR11I and PIP 11I have essentially identical ion crystal packing. This is not the case, however, for PIP11I and MORP11I. The phase behavior and thermal stability of the three salts have also been compared to examine the influence of crystal packing on thermal properties.