The effects of hydration and dehydration of silica-supported vanadia have been investigated with the aim of understanding how these processes alter the structure of the dispersed vanadia. Samples containing either 9 or 12 wt % V2O5/SiO2 were examined by in situ Raman spectroscopy during hydration in 3 kPa water vapor at room temperature and during dehydration at temperatures between 298 and 773 K. The vanadia in freshly dehydrated 9 wt % V2O5/SiO2 is present exclusively in the form of monovanadate species. Monovanadate species are predominant in the 12 wt % V2O5/SiO2, but a small amount of V2O5 is present as well. Room-temperature hydration causes a progressive loss of the Raman band at 1043 cm(-1) characteristic of isolated monovanadate species, and the gradual appearance of bands at 1021, 986, 895, 773, 706, 666, 512, 415, 325, 267, and 158 cm(-1) characteristic of a hydrated vanadia gel. Dehydration at elevated temperatures decomposes the gel and partially restores the presence of isolated monovanadate species. V2O5 particles are also formed during dehydration. Repeated low-temperature hydration and high-temperature dehydration leads to an irreversible conversion of isolated monovanadate species into V2O5 particles. A mechanism by which this process occurs is proposed.