Abstract Thermal and transport properties of Na 2 SO 4 ‐Fe 2 O 3 composites, with molar composition in the range x = 0.1‐0.5, were studied by using impedance spectroscopy and thermal analysis. The electrical conductivity of this solid solution increases rapidly with increasing Fe 2 O 3 content, from a value at about 0.05 × 10 –4 S/cm (for x = 0.0) up to 0.22 × 10 –4 S/cm for x = 0.1. The largest enhancement in the conductivity is observed for 10, 20 and 40 mol% Fe 2 O 3 compositions, a feature not commonly encountered in known composite systems. The step change in the conductivity of pure sodium sulphate at 241 °C, associated with a first order phase transition, that not only shifts slightly to a higher temperatures when the Fe 2 O 3 content increases, but it is also observed a reduction of the step hight. The results are discussed considering an approximation effect of the nanoparticles on the Na+ transport in the Na 2 SO 4 compound. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)