Cs-133 NMR and 2D-EXSY NMR and V-51 NMR spectra have been recorded of CS2S7O7 and of the catalytically important Cs2S2O7-V2O5 mixtures in the temperature range 20-550 degrees C, covering both the solid and liquid region. A high mobility of the Cs+ cations in Cs2S2O7 was observed below the melting point (461 degrees C), and two crystalline modifications seem to be present depending on the method of preparation of CS2S2O7 Cs-133 and V-51 NMR spectra of the Cs2S2O7-V2O5 mixtures with the molar composition Cs/V = 2 and 4 show, by heating from room temperature, in both cases, that the glassy samples transform to crystalline Cs-4(VO)(2)O(SO4)(4) around 220 degrees C and that further heating of the samples up to the melting point leads to a mobility of the Cs cations similar to what was found for Cs2S2O7 For a sample with the composition Cs/V = 1 a similar transformation from a glassy state at around 250 degrees C is observed; however, here the possible compound CsVO2SO4 seems to be transformed to another crystalline modification by heating to 400 degrees C. At 550 degrees C in the molten state the V-51 NMR spectra indicate that the coordination of vanadium changes from distorted octahedral in the solid to tetrahedral in the liquid state.