A series of poly(ethylene glycol)-polyurethane (PEG-PU)/sodium perchlorate (NaClO4) solid electrolytes were prepared, and their properties were characterized with Fourier transform infrared spectroscopy, differential scanning calorimetry, complex impedance analysis, and atomic force microscopy. Results showed that the oxygen atoms of carbonyl and ether oxygen groups had different activities on cations. Both carbonyl and ether oxygen groups participated in the ionic-transport process in PU-based electrolytes. There existed a coordination competition between sodium cations and different oxygen atoms in soft and hard segments of PU. For the PEG-PU/NaClO4 system investigated, amorphous regions and interfacial regions between the amorphous and microcrystalline phases were responsible for ionic conduction. A new ionic-transport mechanism, based on the existence of conduction pathways not only in amorphous regions but also in interfacial regions of microphase-separated PU-based electrolytes, is sketched. Moreover, at a particular concentration of doped salt (EO/NaClO4 12), the PEG-PU/NaClO4 complex revealed a phase-transition point in the morphology and exhibited minimum apparent activation energy and maximum ionic conductivity.