Solid polymer electrolyte films of polyvinyl alcohol (PVA) doped with different amounts of potassium iodide (KI) were prepared using solution cast technique. Formation of complex structure between PVA polymer and KI salt was confirmed by Fourier transform infrared (FTIR) spectroscopy. The changes of crystallization nature and transition temperatures of the solid polymer electrolyte films were investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The optical absorption spectra in the wavelength range from 200 to 800 nm were measured and analyzed in terms of absorption formula for noncrystalline materials. The optical band gap energy, refractive index, and dielectric constants were investigated and showed a clear dependence on the KI content. The optical band gap (both direct and indirect) was determined and explained on the bases of incorporation of charge transfer complexes by the dopants. The calculated values of refractive index and the dielectric constants of the solid polymer electrolyte films increased with increasing KI content.