Measurements of the glass transition temperature (T-g) and free volume behaviour of poly(acrylonitrile) (PAN) and PAN/lithium triflate (LiTf), with varying salt composition from 10 to 66 wt% LiTf, were made by positron annihilation lifetime spectroscopy (PALS). Addition of salt from 10 to 45 wt% LiTf resulted in an increase in the mean free volume cavity size at room temperature (r) as measured by the orthoPositronium (oPs) pickoff lifetime, tau(3), with little change in relative concentration of free volume sites as measured by oPs pickoff intensity, I-3. The region from 45 to 66 wt% salt displayed no variation in relative fi ee volume cavity size and concentration. This salt concentration range (45 wt% < [LiTf] < 66 wt%) corresponds to a region of high ionic conductivity of order 10(-5) to 10(-6) S cm(-1) at T-g as measured by PALS. A percolation phenomenon is postulated to describe conduction in this composition region. Salt addition was shown to lower the T-g as measured by PALS; T-g was 115 degrees C for PAN and 85 degrees C for PAN/66 wt% LiTf. The T-g and free volume behaviour of this polymer-in-salt electrolyte (PISE) was compared to a poly(ether urethane)/LiClO4 where the polymer is the major component, i.e. traditional solid polymer electrolyte (SPE). In contrast to the PISE, the T-g of the SPE was shown to increase with increasing salt concentration from 5.3 to 15.9 wt%. The relative free volume cavity size and concentration at r were shown to decrease with increasing salt concentration. Ionic conductivity in this SPE was of order 10-5 S cm(-1) at r, which is over 60 degrees C above T-g, 10(-8) S cm(-1) at 25 degrees C above T-g, and conductivity was not measurable at T-g. (C) 2000 Elsevier Science Ltd. All rights reserved.