Positron lifetime spectroscopy has been applied to study the temperature dependence of free-volume properties in a solvent-free polymer-salt complex polyethylene oxide (PEO) doped with ammonium iodide (NH4I, with NH4+/EO approximate to 0.076) in the temperature range of 298-353 K. The observed lifetime spectra were resolved into three components and the longest lifetime, tau(3), was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by the free volume. The lifetime component, tau(3), and its intensity, I-3, both showed a significant variation with temperature, which followed a different course in the heating and cooling cycle. Changes in the temperature coefficient of tau(3) and I-3 were observed at T approximate to 328 K, the melting point of the sample. This behaviour is correlated to the temperature variation of the electrical conductivity.