Dual-phase polymer electrolytes that possess good mechanical strength and high ionic conductivity were prepared by mechanically mixing a poly(acrylonitrile-co-butadiene) rubber (NBR) and poly(styrene-co-butadiene) rubber (SBR) binary solution and casting polymer blend films. The films were swollen with lithium salt solutions (e.g. 1 M LiClO4 in gamma-butyrolactone) to obtain dual-phase polymer electrolyte films. As the mixing rate increases, the average domain size of the NBR and SBR in the film decreased, levelling off at a mixing rate of >10 000 rev min(-1) as it approached the value of 5 mu m. A mechanically strong film was obtained by reducing the domain size to less than one-fifth of the film thickness. On the other hand, the ionic conductivity depended on the fraction of NBR in the NBR/SBR matrix rather than on the domain size of the him. Thus high ionic conductivity (>10(-4) S cm(-1)) could be achieved with an NBR weight fraction of over 50% (w/w). Additionally, transmission electron microscope observation and differential scanning calorimetric analysis showed evidence that a dual-phase structure was created, in which the NBR phase provided an ion-conductive pathway and the SBR acted as a mechanically supportive matrix. Quantitative analysis of ionic conductivity suggested that a ’free’ lithium salt solution absorbed in the matrix caused the high ionic conductivity of the polymer electrolyte.