The electrical conductivity and density of pure molten thallium sulphate and the conductivity of TlMeSO 4 , where Me = Li, Na, K, Rb, or Cs, have been measured between 700 and 1150 °C. We have also measured the densities of four equimolar sulphate mixtures and their pure components. The excess volumes were less than 0.5%. The deviations from ideality for the conductivities and their Arrhenius activation energies have been compared with other sulphate systems. The electrical conductivity of a pure univalent sulphate depends both on the radius of the cation and on its mass. In mixtures, there are large negative excess conductivities and positive excess activation energies, when we have cations of different sizes.