The crystal structure of the new Li5.5Ce12F 50 compound has been fully characterized by single-crystal and synchrotron powder X-ray diffraction. An accurate pseudotetragonal structure was described In the monoclinic P21 space group with 68 independent crystallography sites. The Li5.5Ce12F50 composition belongs to the Li2+xCex 3+Ce 12-x 4+F60 solid solution. Its structure consists of an opened fluorine framework where a channel network allows the intercalation of relatively mobile lithium cations, inducing the formation of the mixed-valence cerium (the intercalation of Li+ leads to the reduction of a part of Ce4+ to Ce3+). One part of the lithium ions, necessary for the electroneutrality of the tetravalent equivalent cerium fluoride (Li2Ce12F50 composition), is in a locked fluorine polyhedron. Only the supplementary x amount of lithium is able to be exchanged in Li2+xCex 3+Ce 12-x 4+F50. The structure of Li 2+xCex 3+Ce12-x 4+F 50 is a rearrangement, due to lithium intercalation, of the base CeF4 structure. Bond valence calculation on Ce sites, Ce coordination polyhedra volumes, and a calculated Ce cationic radius give the indication of a partial long-range ordering of trivalent and tetravalent cerium cations In specific slabs of the structure. 7Li NMR spectroscopy and XPS analyses have confirmed all of the structure details. © 2009 American Chemical Society.