With the aim of developing ion-conducting solid polymer electrolytes that combine high ionic conductivity with good mechanical properties, we prepared and investigated nanocomposites of LiClO4-doped ethylene oxide-epichlorohydrin (EO-EPI) copolymers and nanoscale cellulose whiskers derived from tunicates. We show that homogeneous nanocomposite films based on EO-EPI copolymers, LiClO4, and tunicate whiskers can be produced by solution-casting THF/water mixtures comprising these components and subsequent compression-molding. The Young's moduli of the nanocomposites thus produced are increased by a factor of up to >50, when compared to the copolymers, whereas the electrical conductivities experience only comparably small reductions upon introduction of the whiskers. The nanocomposite with the best combination of conductivity (1.6 X 10(-4) S/cm at room temperature and a relative humidity of 75%) and Young's modulus (7 MPa) was obtained with a copolymer having an EO-EPI ratio of 84:16, a whisker content of 10% w/w, and a LiClO4 concentration of 5.8% w/w. (C) 2004 Wiley Periodicals, Inc.