The electronic structure and magnetic properties of the full-Heusler alloys Co2TiX (X element from groups III, IV and V) were studied by first principle calculations. Previous calculations found Co2TiAl and Co2TiSn not to be half metallic. In this paper, however, it will be shown that the alloys with X=Al,Si,Ge,Sn are half metallic and ferromagnetic. The effect of atomic disorder in the Ti-Al sublattices reduces the half metallicity of the Co2TiAl Heusler alloy. (c) 2005 American Institute of Physics.