The oxide Mo4VO14, in its active state, is a partly reduced material with an "amorphous" structure. Temperature-programmed methods reveal that the active lattice oxygen gives rise to a desorption signal at 500 K, which is at the lower end of the temperature window for activity in the selective oxidation of n-propanol. The phase is metastable with respect to oxidative crystallisation into isotropic crystals of the title structure occurring at 781 K. Photoemission revealed in the zero bandgap semiconductor oxide of the amorphous state the MO6 octahedra to be distorted and the oxygen pumping activity to be related to oxidation-reduction cycles of metal d-states situated close to the Fermi energy. Structural studies by XRD and HRTEM showed that the oxygen-pumping state is a nanocrystalline modification of the title structure. The relation of the structural motif to the catalytic function is discussed for this compound and for the whole family of systems in the framework of the theory of glass formation.