The phase diagram of the catalytically important K2S2O7-V2O5 system has been investigated by means of conductometric and NMR spectroscopic methods up to 500 degrees C. From the marked change of the specific conductivity by change of phase, found for 14 different compositions of the K2S2O7-V2O5 binary system, the phase transition temperatures have been obtained. In the V2O5-rich region, where glass formation is pronounced, K-39 NMR spectra of six different compositions showed a marked change of the line width at the liquidus temperatures. The phase diagram based on the combined results exhibits three maxima corresponding to the formation of compounds with the stoichiometry 3K(2)S(2)O(7) . V2O5, 2K(2)S(2)O(7) . V2O5, and 1K(2)S(2)O(7) . V2O5 and melting temperatures of 352, 398, and 425 degrees C, respectively. Three eutectics were found at the compositions X-V2O5 = 0.17, 0.27, and 0.39 with melting temperatures of 314, 348, and 366 degrees C, respectively. The measured conductivities in the liquid region of the 14 different compositions have been fitted to polynomials of the form kappa = A(x) + B(x) (t - 450) + C(x)(t - 450)(2) + D(x)(t - 450)(3), The phase diagram may prove to be useful for the design of new SO2 oxidation catalysts.