N,N-Dimethylpyrrolidinium hydroxide (P<sub>11</sub>OH·4H<sub>2</sub>O) was found to exhibit high ionic conductivity in the solid state (7 × 10<sup>−3</sup> S cm<sup>−1</sup> at 25°C) and unusual thermal properties, and <sup>2</sup>H solid state NMR measurements indicate liquid-like mobility of the deuterium species in the solid state of P<sub>11</sub>OD·4.5D<sub>2</sub>O.<br>