Polymer electrolytes based on a mixed polymer matrix consisting of poly(ethylene glycol) (PEG) and cyanoresins with lithium salt and plasticizer were prepared with an in situ blending process to improve both the mechanical properties and the ionic conductivity (sigma). The PEG/lithium perchlorate (LiClO4) complexes, including blends of cyanoethyl pullulan (CRS) and cyanoethyl poly(vinyl alcohol) (CRV), exhibited higher sigma's than a simple PEG/LiClO4 complex when the blend compositions of CRS/CRV were 5 : 5 or 3 : 7 or than CRV alone. When the CRS/CRV blend was compared with a copolymer of cyanoethyl pullulan and cyanoethyl poly(vinyl alcohol) (CRM) in the same molar ratio, the sigma values of the polymer electrolytes containing the CRM copolymer series were slightly higher than those of the CRS/CRV blends containing PEG/LiClO4 complexes. Moreover, the addition of cyanoresin to PEG/LiClO4/(ethylene carbonate-propylene carbonate) polymer electrolytes provided better thermal stability and dynamic mechanical properties. (c) 2006 Wiley Periodicals, Inc.