The ternary phase diagram Al2O3-SiO2-La2O3 was investigated at 1300 degrees C. The samples were prepared by wet chemical route (sol-gel synthesis) and characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), and EDX microprobe analysis. The XRD powder pattern data of La14Si9O39 compound, as obtained by us, reconfirm the structure proposed in the literature, though with significantly different lattice parameters. No truly ternary compound was found to form at 1300 degrees C. The La14Si9O39 compound extends its stability range in the interior of the phase diagram, forming a solid solution of the type La14+x/3Si9-x AlxO39 which is stable from x = 0 to x = 1.5. This substitutional stoichiometry can be described as a tetrahedral Al for Si substitution on the 6h position and contemporary occupation of vacant La sites.