The crystal structures of the lanthanum and neodymium gallates and yttrium aluminates, doped with neodymium and erbium were investigated by means of X-ray structure analysis (full profile Rietveld refinement, powder diffractometers HZG-4 and DRON-4, Cu and Co K α -radiation). A comparative crystallochemical analysis of the studied structures was performed. All examined compounds belong to the orthorhombically distorted GdFeO 3 -type of structure, space group Pbnm, Z=4. It is shown that the structure deformation parameters such as the rare earth cation shift, the bond length distortion of the polyhedra, and the tilt angles of octahedra increase in the sequence: LaGaO 3 -NdGaO 3 -YAlO 3 :Nd-Y 0 . 5 Er 0 . 5 AlO 3 .