The adsorbed state of NH3 in the vanadium pentaoxide hydrate, V2O5 . nH(2)O, and its chemical reactivity with additionally introduced H2O were studied by gravimetric adsorption measurements, Fourier transform infrared (FT-IR) spectroscopy, mass spectroscopy, and X-ray defraction (XRD) analyses. It was confirmed by FT-IR spectroscopy that NH3 molecules are adsorbed at first to form NH4+ by reacting with intrinsically present H2O and secondly as molecular NH3 or IR-inactive NH4+. These adsorbed states were also confirmed by mass spectroscopy. When H2O vapor is introduced in the NH3-presorbed sample, NH4VO3 is formed topotactically with its(001) plane parallel to that of V2O5 . nH(2)O. Here, H2O reacts with additional molecular NH3 giving locally the NH3 water between the layers. The NH3 water led to the dissolution of the hydrate and to crystallization of NH4VO3.