Two new compounds, LiBi4Nb3O14 and LiBi4Ta3O14, have been synthesized by the solid-state method, using Li2CO3, Bi2O 3, and M2O5 (M = Nb, Ta) in stoichiometric quantities. These compounds crystallize in the monoclinic C2/c space group with a = 13.035(3) Å, b = 7.647(2) Å, c = 12.217(3) Å, β= 101.512(4)°, V = 1193.4(5) Å3, and Z = 4 and a = 13.016(2) Å, b = 7.583(1) Å, c = 12.226(2) Å, β= 101.477(3)°, V = 1182.6(5) Å3, and Z = 4, respectively. These are isostructural and the structure along the b axis consists of layers of [Bi 2O2]2+ units separated by layers of LiO 4 tetrahedra and NbO6 octahedra hence depicting an unusual variation in the Aurivillius phase isolated for the first time. The presence of lithium has been confirmed by 7Li NMR studies, ac impedance measurements and variable temperature 7Li NMR studies indicate oxygen ion conductivity in these materials. The UV-visible spectra suggest a band gap of 3.0 eV for LiBi4Nb3O14 and 3.5 eV for LiBi4Ta3O14, respectively, and the associated studies on degradation of dyes and phenols render these materials suitable for photocatalysis. © 2005 American Chemical Society.