Site preference of cobalt and iron atoms substituted in Ni-MnGa alloys is examined using the compositional dependence of saturation magnetization determined at 5 K. Variations in magnetization of Ni 50 Mn 25þx Ga 25ÀxÀy M y (M ¼ Co, Fe) alloys are systematically compared with the values computed for different possible configurations of site occupancies. The study reveals that the dopant atoms occupy the regular Mn site, rather than the vacant Ga site, with ferromagnetic exchange relative to the moments on Ni and Mn sublattices. The excess and displaced Mn atoms occupy the vacant Ga site with antiferromagnetic exchange. Site preference appears to have a correlation to the atomic volume of the elements. [4][5][6] has studied in detail the structural and magnetic properties of Ni-Mn-Ga alloys system with a focus on the composition dependence of transformation temperatures. The magnetic moment in Ni-Mn-Ga Heusler alloy is mainly localized on the Mn sites, as confirmed by both experiments [7,8] and theoretical calculations [9]. From magnetization measurements, Webster et al. [7] reported that the total saturation magnetic moment per formula unit (m B /f.u.) of Ni 2 MnGa is 4.17. The magnetic moment is localized primarily on the manganese atoms with m Mn $ 3.5 m B , while the moment on nickel atoms is quite small with m Ni $ 0.33 m B . From polarized neutron scattering and magnetization studies, Brown et al.