A symmetry analysis of trigonal, cubic, and tetragonal superstructures of the M2X type that are formed in strongly nonstoichiometric compounds MXy with the B1 structure has been performed. The chan nels of disorder–order transitions MXy M2X have been determined. It has been demonstrated that, in nonstoichiometric carbides and nitrides MXy of Group IV transition metals, a decrease in the temperature can lead to three physically allowable sequences of transformations associated with the formation of M2X ordered phases.