A symmetry analysis has been made of monoclinic and trigonal superstructures of the M6C5 type, which are formed in nonstoichiometric cubic carbides MCy with a B1 structure. Channels of disorder–order transitions MCy M6C5 have been revealed, and the distribution functions of carbon atoms in the M6C5 superstructures have been calculated. The atomic–vacancy ordering in nonstoichiometric cubic carbides of vanadium VCy and niobium NbCy has been investigated using neutron diffraction and X ray diffraction anal yses. It has been shown that, as the temperature decreases, the Group V transition metal carbides MCy can undergo two physically admissible sequences of transformations associated with the formation of M6C5 phases.