An X-ray powder diffraction study of the phase formation in the system V2O5/Nb2O5 is performed. Freeze-dried ammonium vanadate and ammonium oxalato niobate, alkoxide-derived xerogels and a mixture of active oxides are used as precursors to compare the resulting phase composition. Thermal decomposition of the freeze-dried precursor is monitored with DTA/TG and mass spectrometry. In the quasi-binary system V2O5-Nb2O53 metastable VNbO5, V4Nb18O55, VNb9O25 and solid solutions Of V2O5 in TT-Nb2O5 as also thermodynamically stable VNb9O25 exist. The thermal decomposition of freeze-dried vanadate-oxalatoniobate solution allows the synthesis of all these phases in a relative simple manner. Structural relationships between an intermediate phase and the product, or, in the case of solid-state reactions, between one of the starting oxide and the product, favour the desired reaction. Therefore, the structure of a former phase influences or directs the structure of the product similar to a topotactic reaction. (C) 2008 Elsevier Ltd. All rights reserved.