Composite solid electrolytes (C4H9)4NBF4–Al2O3 are synthesized in a wide range of composition. Their thermal, structural, and conducting properties are studied. The conductivity of the composites passes through a maximum at x ~ 0.9 and reaches 7.2 × 10–4 S/cm at 150°C for the 0.1(C4H9)4NBF4–0.9Al2O3 composite. The absence of thermal effect at the melting temperature of the ionic salt and the absence of peaks of tetrabutylammonium tetrafluoroborate in the diffraction pattern for a composite, which exhibits a high ionic conductivity, indicate that at x ≥ 0.9, (C4H9)4NBF4 is in the amorphous state, and ionic transfer occurs along the ionic salt/oxide interface.