A study of the half-metallic character of the semi Heusler alloys Co1−xCuxMnSb (0⩽x⩽0.9) is presented. We investigated the saturation magnetization MS at temperatures from 5K to room temperature and the temperature dependence of the DC magnetic susceptibility χ above Curie temperature TC. The magnetic moments at 5K, for most compositions are very close to the quantized value of 4μB for Mn3+ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with MS (5K)=3.78μB/f.u. These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material. The Curie temperature is decreased from 476K to about 300K as the Cu content increases from 0% to 90%. Above TC, the χ−1 vs T curves follow very well the Curie–Weiss law. The effective moment μeff and paramagnetic Curie temperature θ are derived. A comparison between the values of MS at 5K and μeff shows a transition from localized to itinerant spin system in these compounds.