Modulated differential scanning calorimetry (MDSC) and thermogravimetric analysis (TGA) were used to study the thermal properties of the solid electrolyte composites (1-x)CsHSeO4-xFe2O3 in the concentration range x=0.0–0.5. The pure CsHSeO4 shows two endothermic anomalies at 85 and 128∘C, respectively, on the first heating run. Both anomalies are affected by the Fe2O3 content. The results are explained in terms of approximation effect of the nanopowders with CsHSeO4.