A series of Heusler alloys, with composition Ni2Mn(MnxSn1−x) has been studied. X-Ray and neutron diffraction techniques have been used to investigate the chemical order. Well-ordered alloys were prepared with compositions corresponding to x = 0.1. 0.2, 0.3, and 0.4. The magnetic properties of these ordered alloys indicate that the Mn atoms, which substitute for Sn atoms, are coupled antiferromagnetically to the ferromagnetic manganese sublattice.