We report on the magnetic, magnetotransport and structural characterization of (Ca1-yNdy)2Fe1+xMo 1-xO6 (x<0.5) ferromagnetic double perovskites. It is found that the presence of an excess (x>0) of Fe ions in the metallic sublattice produces a remarkable increase, by more than 90 K, of the Curie temperature. Mössbauer spectroscopy data indicate a reinforcement of the magnetic interactions. We argue that this dramatic enhancement of the ferromagnetic order is due to the strong antiferromagnetic superexchange coupling between near-neighbour Fe-Fe occupying regular and antisite positions in the structure. Moreover, the results indicate that the excess of magnetic ions (Fe) is essential to overcome the dilution effects caused by antisite defects. © 2005 IOP Publishing Ltd.