Dual-phase polymer electrolytes (DPE) that have high ionic conductivity (> 10(-3) S/cm) and good mechanical strength were prepared by mixing NBR and SBR latices and casting films. The latex films absorbed large quantities of lithium salt solution (e.g., 1M lithium perchlorate in gamma-butyrolactone) to obtain DPE films but did not dissolve with swelling. The NBR phase is polar and was impregnated selectively with the polar lithium salt solution, whereas the SBR phase is nonpolar and formed a mechanically-supportive matrix. Transmission electron microscopic (TEM), electron energy loss spectral (EELS), and energy-dispersive x-ray (EDX) analyses showed microscopically the dual-phase structure. Evidence for swelling by lithium salt solution was found only in the NBR phase and not in the SBR phase by EDX microanalysis. Ionic conductivity as a function of NBR content or swelling degree showed clearly that a percolation threshold for ionic conductivity exists.