Ambient temperature conductive plastic crystal phases of alkylmethylpyrrolidinium trifluoromethanesulfonyl amide (TFSA) salts are studied using positron annihilation lifetime spectroscopy (PALS) to examine the role of vacancy size and concentration in conductivity. The ethyl methylpyrrolidinium TFSA salt (P-12 TFSA) has larger vacancies and a greater concentration of vacancies than the dimethylpyrrolidinium TFSA salt (P-11 TFSA) over the temperature range investigated. The relative vacancy size and concentration vary with temperature and reflect the solid-solid transitions as measured by differential scanning calorimetty (DSC). P-12 TFSA has greater conductivity than P-11 TFSA and has furthermore been observed to exhibit slip planes at room temperature. P-12 TFSA has greater entropy changes associated with solid-solid phase transitions below the melting point than P-11 TFSA possibly indicating greater rotational freedom in P-12 TFSA. These results support the notion that the diffusion, conduction, and plastic flow properties of the pyrrolidinium TFSA salts are derived from the lattice vacancies. Crown Copyright