[en] Attempts have been made to prepare crystals of solid solution series having block structures with building elements of [3x4] [4x4], [4x5], and [5x5] octahedra in the system Nb₂O₅/WO₃/NbO₂ by chemical transport reactions with various transport agents. Crystals of about 6 mm in length of these compounds, however, have been prepared by a nearly isothermal chemical transport with HgCl₂ as a mineralizer. The compositional limit of the four solid solution series has been somewhat extended compared to reported values. (author)