A variety of lithium ion conducting borosiloxane polymers were prepared in high yield. The temperature dependence of ionic conductivity was fit well by the Vogel–Tammann–Fulcher equation, indicating that ion motion was strongly coupled to segmental motion of oligoether side chains. The borosiloxane electrolytes also exhibited large fractional free volume at the glass transition temperature, which led to enhanced ionic conductivity compared to electrolytes incorporating high molecular weight poly(ethylene oxide). The cationic transference number was enhanced due to anion interaction with the Lewis acidic sites on the polymer framework.