The systems K2S2O7-V2O5 and K2S2O7-K2SO4-V2O5 are important for the catalytic oxidation of SO2 to SO3. The enthalpies of mixing of solid V2O5 and liquid K2S2O7 have been measured in the whole liquidus range, i.e. up to a mole fraction of around X(V2O5) = 0.5 : at 400 and 450 degrees C. The enthalpy of liquid - liquid mixing could then be calculated and Delta H-mix was found to be strongly exothermic, the partial enthalpy at infinite dilution in K2S2O7, Delta H-V2O5(infinity)$($) over bar$$,being -85.0 kJ mol(-1) at 450 degrees C. The local minimum for Delta H-mix found at X(V2O5) = 0.33 indicates the formation of a dimeric complex (VO2)(2)(SO4)(2)S2O74- (or possibly formulated as (VO)(2)O(SO4)(4)(4-)) over the whole temperature range 400-450 degrees C. However, lowering the temperature in this range apparently increases the formation of polymeric complexes such as (VO2SO4)(n)(n-). The enthalpy of liquid-liquid mixing at 400 and 450 degrees C has also been obtained for K2SO4 as well as for the three cross-sections K2S2O7.V2O5, 2K(2)S(2)O(7).V2O5 and 3K(2)S(2)O(7).V2O5 of the binary system. In these pseudo-binary systems dimeric and polymeric complexes such as (VO2)(2)(SO4)(3)(4-) and (VO2(SO4)(2))(n)(3n-) are also probably formed in which polymerization seems to increase at lower temperatures. With increased addition of K2SO4, a compound, probably K3VO2(SO4)(2), precipitates. Finally, the conductivities of the same three pseudo-binary cross-sections have been measured in the temperature range 390-500 degrees C and at 20 totally different compositions in the range X(K2SO4) = 0-0.55. For each composition and temperature T in the measured range, the conductivity can be expressed by an equation of the form kappa = A(X) + B(X)(T - 450) + C(X)(T - 450)(2) + D(X)(T - 450)(3). The calculated molar conductivities exhibit a large deviation from ideality, supporting the concept of strong interaction between the components of the melts.