The free volume behaviour of the polyacrylonitrile/lithium triflate system is investigated over the composition range 0-75 wt % salt. The addition of salt, up to 45 wt %, to the PAN polymer substantially increases the free volume as measured by the orthopositronium pickoff lifetime, tau(3). Beyond this salt concentration (i.e., 45-70 wt %) the free volume remains approximately constant. This constant free volume region corresponds to a region of high ionic conductivity in the glassy state, making these materials polymer-based fast ion conductors, that is, having a decoupling ratio R-tau much greater than 1. The high salt content in these fast ion conductors results in a high susceptibility to polar solvents such as water. For all compositions, water absorption results in a free volume increase attributed to plasticization; however, in the fast ion conducting region, a significantly larger free volume response due to plasticization is measured and may be connected to a percolation morphology in these samples. Salt addition is shown to lower the T-g, as measured by positron annihilation lifetime spectroscopy (PALS). T-g is 115 degrees C for PAN and 85 degrees C for 66 wt % lithium triflate.