AbstractAn expression for the order–disorder transformation temperature in substitutional alloys of AB3‐type is determined in terms of the classical theory of ordering in alloys with low impurity concentration. The order–disorder transformation temperature is shown to increase at the initial stages of alloying at any relations of ordering energies. The data on theoretical calculations are compared with experimental ones obtained with Ni3Fe alloy doped by different impurities. Much consideration is given to results obtained with Ni3(Fe1−xMnx) alloys. Using neutron diffraction analysis, the lack of Ni24Fe4Mn4ternary superstructure is shown, and theTK(x) dependence with the extremum atx≈ 0.4 is obtained from dilatometric and specific heat me