The phase diagram of the catalytically important K2S2O7-KHSO4 solvent system has been investigated by means of electrochemical, thermal, and spectroscopic methods. The phase diagram exhibits a eutectic at X(KHSO4) = 0.94(1) with a temperature of fusion of 205 degrees C. No compound is formed in the system, but the strong alpha --> beta solid-solid transition of K2S2O7, found at 318 degrees C with Delta H-tr = 21.8 kJ/mol, gives rise to a marked change in the slope of the liquidus curve at this temperature. The experimental phase diagram is in very good accordance with a calculated diagram based on the assumption of an ideal liquid mixture. K-39, H-1, O-17, and S-33 NMR measurements on the molten K2S2O7-KHSO4 mixtures up to 540 degrees C show that a fast ionic exchange takes place in the melt at all compositions. The conductivities of the solid and molten K2S2O7-KHSO4 systems were measured at 13 different compositions in the whole composition range, X(KHSO4) = 0-1 For each composition in the temperature range examined, the conductivity of the molten mixtures has been expressed by equations of the form kappa = A(X) + B(X)(T - 600) + C(X)(T - 600)(2). The measurements indicate an enhanced molar conductivity of the binary system, probably due to delocalization of the conducting ions compared to the pure molten components.