Multinuclear 23Na, 39K, 133Cs, 17O, 51V magnetic resonance studies of the M 2S 2O 7-V 2O 5 (M=Na, K, Cs) systems in the temperature range 20–650°C have been performed for vanadium oxide mole fractions, X(V 2O 5), in the range 0–0.5. At ambient temperature the melt-quenched glassy samples exhibit a three-dimensional network of vanadium oxosulfate complexes. Octahedral coordination of vanadium atoms is found in the glassy samples at all compositions studied, in accordance with 51V NMR spectra. Alkali cations are distributed randomly within an anion network. At high vanadium concentration the structure of vanadium sites in the glasses is very similar to that found in Cs 4(VO) 2O(SO 4) 4, whereas for small vanadium contents the vanadium sites are separated by additional sulfate ligands. Heating to the glass-transition temperature, T g, and above, leads to jumps of the alkali cations between different sites. The mobility of pyrosulfate groups is accompanied by dissociation to SO 4 2− and SO 3. At the elevated temperature the mobility of SO 3 molecules is sufficient to participate in chemical exchange with the sulfate groups of the network. Addition/splitting mechanism involving SO 3 has been proposed to be responsible for random fluctuations of the 51V nuclear quadrupole tensor at given vanadium network sites with characteristic correlation time τ c. For 10 −8< τ c<10 −6 s the 51V NMR line became unobservable. For Cs-containing samples the increase of the temperature is accompanied by fast crystallization. In this case a cooperative motion of the anion network, caused by bond breaking and bond formation, dominates at temperatures around T g. The NMR spectra of alkali metals were found to be very characteristic for the structure of the network formed in melts between V 2O 5 and M 2S 2O 7. 17O, 23Na, 39K, 133Cs spectra recorded at 500°C point to the formation of different species and rapid exchange between them. A change of the local vanadium environment in melts takes place at X(V 2O 5)∼0.1 and 0.3 most probably due to the formation of dimeric and polymeric V(V) complexes, possibly (VO) 2O(SO 4) 4 4− and (VO 2SO 4) n n− . Correlation time of 51V quadrupole tensor fluctuations for samples with X(V 2O 5)∼0.1–0.5 is higher than 10 −8 s, which makes 51V NMR spectra unobservable in the region 400–500°C, whereas for more dilute samples, τ c is determined mainly by the size of the vanadium-sulfate species making 51V spectra of these samples observable. The dependence of 51V chemical shift on the vanadium concentration indicates a change of coordination number in the system M 2S 2O 7-V 2O 5 from tetrahedral in pure V 2O 5 to octahedral in dilute samples. The structure of supported catalysts is very similar to the structure of bulk melts (M 2S 2O 7-V 2O 5), the main difference revealed is lower mobility of all structural units (such as metal cations, SO 4 2− and SO 3) for the supported melts.