The clathrate-I phase Ba₈Al_xSi_46-x has been structurally characterized at the composition x = 6.9 (space group Pm3¯n, no. 223, a = 10.4645(2) Å). A crystal structure model comprising the distribution of aluminium and silicon atoms in the clathrate framework was established: 5.7 Al atoms and 0.3 Si atoms occupy the crystallographic site 6c, while 1.2 Al atoms and 22.8 Si atoms occupy site 24k. The atomic distribution was established based on a combination of ²⁷Al and ²⁹Si NMR experiments, X-ray single-crystal diffraction and wavelength-dispersive X-ray spectroscopy.