Differential scanning calorimetry (DSC), impedance spectroscopy (IS) and X ray diffraction were used to study the composite solid electrolytes (1-x)NH4HSO4-xAl2O3 (x?0.5) and (1-x)NH₄HSO_4-xSiO₂ (x<=0.7). Alumina with grain size of 10 ?m, pores size of ~6 nm and silica with grain size of 14 nm were used. The SiO2 and Al₂O₃ particles have different effects on the NH₄HSO₄ thermal phase behaviour: the phase transitions of NH₄HSO₄ at 154 K and 420 K (melt-ing point) were not affected by the silica doping, whereas in the (1-x)NH₄HSO_4-xAl₂O₃ com-posites, Al2O3 affects the enthalpy of both transitions such that the phase transitions at 154 K and at 420 K disappeared for x >=0.3. The DSC results shows that new morphological states are formed by the Al2O3 doping because new phase boundaries are observed below the melting point of AHS in the composites. The X-ray diffraction data also confirm that the composites with alumina be-comes more amorphous as x increases reaching completion for x >= 0.3. On the other hand, the dc-conductivity is only improved on adding either Al₂O₃ or SiO₂ to AHS in the solid phases of the composites. The results are discussed in terms of the influence of the dispersed second phase on the overall electrical response and phase behaviour of the composites.