Dual-phase polymer electrolytes (DPE) are analyzed by ac impedance spectroscopy. DPE is prepared from an NBR/SBR latex mixture in the following process : the latex mixture is dried to produce a polymer matrix film, and the polymer matrix is then impregnated with a lithium salt solution as a plasticizer, which selectively permeates into the NBR phase. When a polymer matrix is impregnated with 1M LiClO4/gamma-butyrolactone and its NBR content is small, a distorted complex impedance spectrum, composed of overlapping two semicircles, is observed. The analysis based on an equivalent circuit indicates that isolated fragments of the conductive NBR phase are formulated in addition to the continuous conductive pathways. With increasing content of NBR, the two semicircles get merged. This suggests that the isolated conductive phase comes to be integrated into the continuous ionic pathways. The impregnation process of DPE with a less polar plasticizer was followed by analyzing the complex impedance and modulus spectra. According to this analysis, secondary conductive pathways are formed along the SBR particle boundaries in addition to the NBR conductive phase. Swelling of NBR particles progresses in the secondary conductive pathways, and the NBR particles are finally linked to form continuous ionic pathways between the electrodes.