Composite solid electrolytes in the system (1 − x)β-MgSO4:Mg(NO3)2–x MgO with x = 0.1–0.6 (mole) were synthesized by sol-gel method and analysed by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, energy dispersive X-ray, Fourier transform infrared spectroscopy and alternating current impedance spectroscopy. Structural analysis showed solid-solid phase transformation from anhydrous MgSO4 to β-MgSO4. Thermal study showed melting and decomposition of β-MgSO4 at ∼1030 and ∼1114 °C, respectively, in all composite samples. Morphological analysis indicated crystalline and amorphous phases of particles with average size ranging from 0.1 to ∼5.6 μm. For samples with x ≥ 0.4, β-MgSO4 crystals were not ordered and formation of a new MgO phase due to initial decomposition of β-MgSO4 was observed. The conductivities of the composites were in the order of 10−6 S cm−1 at room temperature and account from the formation of MgO···Mg2+ species at the interface regions between β-MgSO4 and MgO phases.