The plasticizing effect of the K+ ions and succinonitrile on the electrical conductivity of a new redox-couple solid polymer electrolyte system, (1 - x)[0.5poly-(ethylene oxide):0.5succinonitrile]:x[0.91KI:0.1I(2)] with x = 0-0.2 in weight fraction, is reported. An increase of x resulted in an increase of the electrical conductivity (sigma(25 degrees C)) of the electrolyte. The electrolyte with x = 0.15 exhibited the highest sigma(25 degrees C) value, similar to 7 X 10(-4) S cm(-1), and is referred to as the optimum conducting composition (OCC). In addition to an increase of the mobile ion concentration with increasing x, X-ray diffractometry, Fourier-transform infrared spectroscopy, polarized optical microscopy, UV-vis spectroscopy, and differential scanning calorimetry studies revealed a decrease of poly(ethylene oxide) crystallinity/an increase of ionic mobility, indicating the plasticizing effect of the K+ ions. Plasticizing and molecular diffusing properties of the succinonitrile further helped to improve the electrical conductivity of the electrolyte.