The conductivity of the binary system CS2S2O7-CsHSO4 has been measured at 20 different molten compositions in the full composition range and in the temperature range 430-750 K. From the obtained liquidus-solidus phase transition temperatures, the phase diagram has been constructed. It is of the simple eutectic type with the composition X(CsHSO4) = 0.86 and temperature of fusion of 470 K for the eutectic. The previously unpublished temperature of fusion of CsHSO4 was found to be 491.7 K. The experimental phase diagram is in good accordance with a calculated diagram based on measured thermodynamic properties of the pure compounds and the heat of liquid-liquid mixing measured for X(CsHSO4) = 0.5 and 1.0. Cs-113 and O-17 NMR spectra and Raman spectra of the liquid Cs2S2O7-CsHSO4 system indicate the presence of a temperature sensitive equilibrium 2HSO(4) reversible arrow S2O72- + H2O Where the water molecules are strongly associated in the melt. Fast exchange reactions take place between the species present, leading to the observed O-17 NMR single line with an averaged chemical shift. Super-ionic and thermodynamic stable phases and the temperatures of phase transitions have been identified from the NMR measurements on CsHSO4, CS2S2O7, and Cs2S2O7-CsHSO4 mixtures. For 11 selected compositions covering the entire composition range of the CS2S2O7-CsHSO4 binary system, the conductivity of the molten state has been expressed by equations of the form k(X) = A(X) + B(X)(T - T-m) + C(X)(T - T-m)(2) where T-m is the intermediate temperature of the measured temperature range and X is the initial mol fraction of CsHSO4.