The synthesis and characterization of flexible solid polymer electrolytes (SPEs) based on interpenetrating polymer networks (IPNs) are discussed. IPNs were prepared from nitrile butadiene rubber (NBR) and poly(ethylene oxide) (PEO) using a two-step process. The NBR network was obtained by dicumyl peroxide cross-linking at high temperature and pressure. A free radical copolymerization of poly(ethylene glycol) methacrylate and dimethacrylate led to the formation of the PEO network within the NBR network. Polymerization kinetics were followed by dynamic mechanical analysis (DMA) for the NBR network and by Fourier transform spectroscopy in the near- and mid-infrared for the PEO network. IPN mechanical properties, examined using DMA and tensile strength tests, reveal an IPN elongation with a breaking point of 110%. IPN conductivities reach 7.4 x 10(-7) S cm(-1) at 30 degrees C when doped with lithium bis(trifluoromethanesulfonyl)imide (LiTESI). Moreover, IPNs exhibit an ionic conductivity as high as 0.7 x 10(-3) S cm(-1) at 30 degrees C when swollen in N-ethylmethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquid (EMITFSI).