The structure of Er 2RhSi 3 was redetermined by X-ray single-crystal diffraction ( λ(Mo Kα)=0.71073 A ̊ , μ=42.392 mm −1, F(000)=892, T=293 K, R=0.037, wR=0.030 for 244 contributing unique reflections). It is shown that this silicide has a hexagonal structure of the Lu 2CoGa 3 type, hP24, (194) P6 3/ mmc-khfb, a=8.1130(7), c=7.7556(9) A ̊ , V=442.09(9) A ̊ 3, Z=4, M r=521.68, D x=7.838 mg mm −3 . The structure of isotypic Er 2 Co 1.4Ga 2.6 ( hP24, (194) P6 3/ mmc-khfb, a=8.607(3), c=6.898(3) A ̊ , V=442.5(4) A ̊ 3, Z=4, M r=598.30, D x=8.980 mg mm −3 ) was refined by X-ray powder diffraction ( λ(Fe Kα)=1.93735 A ̊ , F(000)=1017.6, T=293 K, R=0.092 for 61 reflections). The cell parameters of R 2CO 1+xGa 3−x phases (R Y, Tb, Dy, Ho, Er, Tm, Yb; x= ±0.4) with the same structure type were obtained from X-ray powder diagrams. The Lu 2CoGa 3 structure is a distorted substitution variant of the A1B 2 type, where the trigonal Lu 6 prisms centred by cobalt atoms share triangular faces in infinite columns. The cobalt atoms are displaced away from the prism centres like the mercury atoms in the orthorhombic KHg 2 structure type. The distortions in Er 2RhSi 3 are of a lesser magnitude than those observed in Lu 2CoGa 3. A progressive substitution of gallium by cobalt along the RGa 2-RCoGa cross-sections of the R-Co-Ga systems (R Y, Tb, Dy, Ho, Er, Tm, Yb, Lu) leads to a step by step deformation of the trigonal prisms. The main features of other deformation and substitution derivatives of the A1B 2 type are discussed.