This paper describes the relationship between the ionic conduction and the characteristics of defects in plastic crystalline phases of N,N'-dimethylpyrrolidinium bis(trifluoromeihanesulfonyl)amide (P(11)TFSA). Positron annihilation lifetime spectroscopy (PALS) shows that the phase III to phase II transition involves the generation and expansion of vacancies, which is confirmed by measurement of volumetric expansion. The vacancies expand to match the cation size in phase II and this gives rise to an increase in the conductivity. The relationship between the vacancy volume and the conductivity obeys a Cohen-Turnbull free volume conduction model. The critical volume matches the volume of a cation-anion pair in phase III, which indicates a Schottky mechanism. The critical volume shows a greater value in phase II, which is probably indicative of a pipe diffusion mechanism. Crown Copyright (c) 2006 Published by Elsevier B.V. All rights reserved.