Powder neutron diffraction analysis was carried out on a sample of Li1+xAlxTi2–xP3O12 at two temperatures, 10 K and 300 K. After heat treatment the material contained small amounts of LiTiOPO4 as impurity. Refinement of the neutron diffraction data confirmed that the main phase crystallized with a NASICON structure. No structural change was observed at low temperature. Al atoms randomly substituted Ti atoms, and additional Li atoms occupied a second site in the crystal structure. A detailed analysis of the diffraction data, including the inspection of Fourier difference maps, indicated that these excess Li+ ions occupied a 6a site within the ion conduction channels of the NASICON-type structure instead of the formerly reported 18e or 36f sites.