The phase relations of the CeCo(9 + δ)Si(4 - δ) system have been studied by means of scanning electron microscopy, electron microprobe analysis and x-ray diffraction. Essentially single phase samples CeCo(9 + δ)Si(4 - δ) (structure-type LaFe(9)Si(4) with space group I4/mcm) are formed in a narrow composition range - 0.3 ≤ δ < 0.1, where stoichiometric CeCo(9)Si(4) exhibits full structural order in space group I4/mcm. The evolution of the ground state of correlated 3d and 4f electrons in the solid solution CeCo(9 + δ)Si(4 - δ) has been investigated by dc susceptibility, magnetization, specific heat and resistivity measurements. Stoichiometric CeCo(9)Si(4) exhibits paramagnetic Kondo lattice behaviour with a largely reduced Co 3d contribution to the magnetic susceptibility as compared to nearly ferromagnetic LaCo(9)Si(4). Nonetheless, very similar to the solid solution LaCo(9 + δ)Si(4 - δ), weak ferromagnetism is observed in CeCo(9 + δ)Si(4 - δ) for δ > 0 and is attributed to substitutional disorder at the Si-sublattice.