[en] Two orthorhombic structures were studied using single-crystal X-ray diffraction data and electron microprobe analyses: Ce(Co_1-xSi_x)Si₂ with x=0.17(1) (a=4.143(1) A, b=16.505(3) A, c=4.076(1) A) and LuCo_1-x-Si₂ with x=0.36(2) (a=3.928(2) A, b=15.888(7) A, c=3.857(1) A). Both crystallize in the Cmcm space group and are non-stoichiometric CeNiSi₂ (or BaCuSn₂)-type phases corresponding to two different types of defects. In LuCo_1-xSi₂, the transition metal site is only partially occupied by cobalt whereas in Ce(Co_1-xSi_x)Si₂ the same site is fully occupied by a mixture of cobalt and silicon atoms. Ce(Co_1-xSi_x)Si₂ compounds annealed at 1273 K have a homogeneity range which has been verified for 0≤x<0.17. However, as-cast LuCo_1-xSi₂ compounds were only obtained in the range 0.15≤x<0.36. (orig.)