New types of poly(ethylene oxide) (PEO) based, gel-type polymer electrolytes were obtained by swelling a composite membrane, formed of linear PEO, LiClO4 salt, and nanoparticle SiO2 filler, by a nonaqueous diethylcarbonate (DEC)/ethylene carbonate (EC) solvent mixture. The role of the molar fraction of two solvents and of the molecular weight of PEO on the swelling characteristics is discussed. An ionic conductivity higher than 10(-3) S cm(-1) at room temperature is obtained for PEO membranes swelled 300 wt % vs. the initial weight by using a 25% molar EC/DEC solvent mixture. Differential scanning calorimetry measurements were also performed in order to determine the status of solvents in these optimized gel membranes. Finally, the validity of this new concept for the formation of homogeneous gel-type polymer electrolytes is evaluated and discussed.