Physico-chemical properties, such as densities, conductivities, enthalpies of phase transitions and melting points, have been measured and summarised for the alkali pyrosulphates Na2S2O7, K2S2O7, Rb2S2O7, CS2S2O7. The densities of the molten pyrosulphates could be expressed by the linear expression rho=A+B(T-T-m) where T-m is the middle temperature of the temperature range measured, i.e. from the melting point and up to 550 degrees C at the maximum. The specific conductivities of the molten pyrosulphates have been expressed by the equation rho=A+B(T-T-m)+C(T-T-m)(2)+ D(T-T-m)(3) in the temperature range from the melting point and up to 500 degrees C at the maximum. From these measurements also the activation energy for the equivalent conductivity of the alkali cations in the melts could be calculated and compared to the analogous alkalisulphates. By calorimetric investigations of the alkali pyrosulphates the temperatures of fusion, the enthalpies and entropies of fusion and possible solid-solid transitions together with the molar heat capacities of the solid and liquid pyrosulphates at 300-800 K, have been obtained and discussed in relation to the conductrometric measurements and the few related previous investigations.