Nonstoichiometric carbides are unique entities for exploring atomicordering. These compounds display a large variety of superstructures withdifferent symmetry. The effect of ordering on all properties ofnonstoichiometric carbides is large and comparable with variation ofproperties in the homogeneity interval of a disordered compound. In thepresent paper the data on order-disorder phase transformations in stronglynonstoichiometric carbides of Group V transition metals are reviewed.The main ordered phase of these compounds is the superstructure of M6C5 type. Taking into consideration recent findings we showed thattrigonal (space group P3112) and monoclinic (space groups C2/m andC2/c) M6C5 superstructures are formed as a result of ordering ofnonstoichiometric cubic (B1 structure) carbides MCy of Group Vtransition d metals. Symmetry analysis of the monoclinic and trigonalM6C5 superstructures is performed, MCy → M6C5 disorder-ordertransition channels are found, and the distribution functions of carbonatoms in M6C5 superstructures are calculated. It is shown that with adecrease in temperature, two physically admissible sequences oftransformaions connected with the formation of M6C5channels phases are possiblein MCy carbides of Group V transition metals. The first sequence MCy(space group Fm3m ) → M6C5 (space group C2/m) → M6C5 (space groupC2/c) includes disorder-order and order-order transformations. Thesecond sequence involves disorder-order transformation MCy (spacegroup Fm3m ) → M6C5 (space group P3112) and a polymorphoustransformation M6C5 (space group P3112) → M6C5 (space group C2/c).