The crystal structure of La10W2O21, which has to be reformulated (La5.667W0.333)LaWO14 square(2), is best described, on average, by a 2 X 2 X 2 anion-deficient fluorite-related superstructure cubic cell, with space group F (4) over bar 3m, Z = 4, and a = 11.17932(6) angstrom, similar to Y7ReO14-delta. The 32 cations are distributed with lanthanum on the 4a-site, tungsten on the 4b-site, and a partial occupancy of the 24g-site by La (94%) and W. The 56 oxygen atoms occupy four 16e-sites, three of them fully and with an occupancy of 1/2 for the fourth one. Others M10W2O21 (M = Er, Y) adopt a 3 X 2 x 2 fluorite superstructure with W in octahedral sites, whereas W is mainly in tetrahedral sites in La10W2O21. Several powerful techniques such as crystal image furnace synthesis, La-139 nuclear magnetic resonance (NMR) and convergent beam electron diffraction (CBED) were used to achieve our results. Transmission electron microscopy (microdiffraction, CBED, and Tanaka patterns) brought us the real symmetry, showing that indeed classical cubic twinning along the 3-fold axis does take place. The surprising La/W mixed site is nicely confirmed by La-139 NMR. This compound exhibits interesting fast oxide ion conducting properties, comparable with LAMOX (Lacorre et al. Nature 2000, 404, 856-858) at low temperature. As opposed to many ionic conductors, no temperature structural transition is observed. Its conductivity is about 6.4 x 10(-4) S.cm(-1) at 700 degrees C.