High-resolution Si-29, Al-27, N-15, and O-17 MAS NMR spectra have been obtained for both glass and crystalline samples of lanthanum U-phase, La3Si3Al3O12N2,. Previous X-ray single-crystal studies have shown that this phase is iso-structural with rare-earth gallogermanates of the type Ln(3)Ga(5)GeO(14), the atomic arrangement consisting of layers of [(Si,Al)(O,N)(4)] tetrahedra in the x, y plane of the hexagonal unit cell, linked together in the z-direction by [AlO6] octahedra, with rare-earth cations occupying the large holes in this network. However, previous studies obtained only a limited amount of information about cation and anion ordering, mainly deduced from bond-length data. NMR spectra have not only enabled the change in structure from glass to crystalline ceramic to be monitored, but have also given detailed ordering information about the latter, indicating partial disorder of both (Si,Al) and (O,N) on their respective sites in the tetrahedra. (C) 2004 Elsevier Inc. All rights reserved.