The catalytically interesting binary system Rb2S2O7-V2O5 has been investigated by means of calorimetric and conductometric methods. Seventeen different compositions of the binary system in the mole fraction range X-V2O5 = 0-0.45 have been investigated, and the phase transition temperatures obtained. Based on these, the phase diagram of the Rb2S2O7-V2O5 system has been constructed in the indicated composition range. The phase diagram exhibits a local maximum at X-V2O5 = 0.33 and probably also at XV2O5 = 0.5. This corresponds to the formation of two compounds Rb-4(VO)(2)O(SO4)(4) and RbVO2SO4, respectively, as found previously in the analogous Cs system. Two eutectics were found at X-V2O5 = 0.175 and X-V2O5 = 0.43, with the temperatures of fusion of 348 degrees C and 397 degrees C, respectively. The conductivities of each of the 15 molten mixtures investigated have been expressed by polynomials of the form kappa = A(X) + B(X)(t - 470) + C(X)(t - 470)(2) + D(X)(t - 470)(3). The conductivity of the Rb2S2O7-V2O5 system decreases by increasing mole fraction XV2O5 in the melt, indicating decreasing ionic character of the melt. A comparison is made with the analogous Na, K, and Cs systems.