The structures of ordered and disordered cubic TixOz= TiOy (y = z/x) phases containing both Ti and O vacancies were studied by x-ray diffraction. The results demonstrate that annealing TiOy with y = 0.9–1.1 below 1300 K leads to the formation of a monoclinic ordered Ti5O5 phase (sp. gr. C2/m (A12m/1)). Symmetry analysis of the monoclinic Ti5O5 superstructure indicates that the TiOy–Ti5O5 ordering transition is defined by the {k10} Lifshits and {k4} and {k1} non-Lifshits wavevector manifolds. The ordering is shown to be a firstorder transition accompanied by a decrease in the unit-cell volume of the parent lattice. The Ti and O distribution functions in titanium monoxide are calculated, and the Ti- and O-site near-neighbor environments in the monoclinic Ti5O5 superstructure are identified. The region of possible long-range-order parameters is outlined.