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The Bragg-Williams approximation is the basis of the model of the order-disorder transitions in Heusler alloys. The formulae for the critical temperatures of the transitions are derived. The quantitative results of the calculations of the critical temperatures are obtained after the application of the Lennard-Jones type model of the pair potential of atoms in a crystalline lattice. The calculated values of the critical temperatures are compared with experimental results and the reasons of the discrepancy between the model and the experiment are discussed.

X‐rays diffraction studies of the processes of ordering of atoms in the Heusler alloys B2MnA1 (B = Cu, Ni, Co, Pd, Pt) indicate that these transitions proceed according to the following scheme: i.e. two transitions occure: from the structure L21 to B2 and from the structure B2 to A2. The values of the critical temperatures of the above transitions obtained experimentally are in sufficient agreement with the theoretical results. There is also a agreement between the measured and calculated values of the lattice constants of the investigated alloys.