Due to the considerable interest in vanadium niobium oxides as a lithium storage material, the kinetics and transformation processes of the V2O5-5Nb2O5 system have been investigated by in situ synchrotron powder X-ray diffraction. The diffraction data after the thermal treatments selected with a view on the most significant features were supplemented with specific ex situ experiments conducted using a laboratory rotating anode X-ray diffractometer. The morphological changes of the mixed powders assuming an amorphous and nanocrystalline solid solution structure as a function of the temperature were inspected by scanning electron microscopy observations. The structural solution of the powder diffraction pattern of the phase recorded in situ at a temperature of about 700 °C was compatible with an orthorhombic crystal structure with the space group Amm2. The obtained lattice parameters for this structure were a = 3.965 Å; b = 17.395 Å, c = 17.742 Å, and the cell composition was V4Nb20O60, Pearson symbol oA84, and density = 4.10 g cm-3. In this structure, while the niobium atoms may be four-, five-, and six-fold coordinated by oxygen atoms, the vanadium atoms were six-fold or seven-fold coordinated. At the temperature of 800 °C and just above, the selected 1 : 2 and 1 : 3 V2O5-Nb2O5 compositions, respectively, returned mostly a tetragonal VNb9O25 phase, in line with earlier observations conducted for determination of the stability phase diagram of such quasi-binary systems.