The free-volumes of solid polymer electrolytes (SPE) were characterized using positron annihilation lifetime (PAL) spectroscopy, FTIR, and scanning electron microscope (SEM) techniques. The SPE based on poly(vinyl alcohol) (PVA) and sodium bromide (NaBr) complexed with sulfuric acid (SA) H2SO4 at different weight percent ratios were prepared using solution cast technique. The PAL results indicate that a higher SA content (more than 0.87 mol/L) in (PVA)(0.7)(NaBr)(0.3) matrix increase the free-volume hole size from 58 angstrom(3) to 87 angstrom(3). The increase in the SPE free-volume with higher SA content was associated with a decrease in the SPE crystallinity. It is postulated that the incorporated SA interrupt polymeric chain packing and retard crystallization during electrolyte films formation. The FTIR spectral studies indicate that the SA content higher than 0.87 mol/L induces chemical modifications within the PVA, which results in chain scission. The PAL study shows that the chain scissions within the polymer matrix affect the free volume hole density (I-3) and hence the microstructure. I3 was found to be decreased from similar or equal to 11 to similar or equal to 6 %, resulting in lower fractional free-volume holes in the SPE films. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2038-2044, 2010