Statistically designed mixture experiments were performed for the ternary electrolyte polymer system poly(ethylene oxide) (PEO) - lithium perchlorate - ethylene carbonate (EC). Ionic conductivities and crystalline/glass transition temperatures were measured for 15 ternary mixtures of varying ingredient proportions. Although conductivity increases with salt concentration, the plasticizer/polymer ratio is important in determining conductivities at higher salt proportions. A quadratic mixture model provides an accurate description of how conductivity at room temperature changes with varying ingredient proportions. It predicts a maximum conductivity for a 0.50 PEO, 0.21 LiClO4, 0.29 EC (weight fractions) mixture. A special cubic model is found to be most appropriate for representing how the corrected transition temperature values depend on ingredient proportions. The temperature dependence of conductivity was measured for some of the mixtures. The same composition exhibited higher conductivities at room temperature and up to 100 degrees C. The VTF parameters for the plasticized systems were compared with those of non-plasticized ones.