We demonstrate that miscibility affects the ionic conductivity of ternary polymer blends of lithium perchlorate (LiClO4), poly(ethylene oxide) (PEO), and poly(E-caprolactone) (PCL). Although individually these three binary blends are fully miscible, a closed immiscibility loop exists in the ternary blend phase diagram as a result of the complicated interactions among LiClO4, PEO, and PCL. The addition of PCL suppresses the crystallization of PEO and results in higher ionic conductivity. FTIR spectroscopy studies indicate that an excess PCL content causes immiscibility, which results in PCL being excluded from the ternary blends. Consequently, the maximum ionic conductivity (6.3 x 10(-7) S cm(-1)) at ambient temperature of ternary blends having a fixed LiClO4 content (25 wt %) is at a composition of 25/60/15 (LiClO4:PEO:PCL).