Orthorhombic and trigonal Mo–V–oxides, which are the some of the most promising selective oxidation catalysts, were produced by the building blocks mechanism. Both oxides were produced by reaction of ammonium heptamolybdate and vanadyl sulfate under hydrothermal conditions. Structural characterization of the orthorhombic and trigonal Mo–V–oxides revealed that both oxides contained pentagonal (M)M₅O₂₁ units. The pentagonal (M)M₅O₂₁ units were also found in the reaction mixture before the hydrothermal reaction. Therefore, assembly of the pentagonal (M)M₅O₂₁ units can occur under hydrothermal conditions to form three-dimensional Mo–V-based metal oxide solids. Based on the building block mechanism, orthorhombic Mo–V–Sb–oxide single crystal large enough for single crystal X-ray analysis was formed, and the first single crystal X-ray structure analysis of the orthorhombic Mo–V-based oxides was obtained. The orthorhombic Mo–V–oxide has an interesting micropore channel, in which micropore volume is continuously and reversibly tunable by redox treatment.