Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula CO(2)MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co1-xMnx](2)MnSi is of particular interest since Mn3Si is known to crystallize in the Heusler L2(1) lattice structure of CO(2)MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts. (c) 2007 Elsevier Ltd. All rights reserved.