The new layered transition metal oxides NaRbLnMO(5) (Ln = Nd, Sm, Eu, Gd; M = Nb, Ta) were synthesized by direct solid-state reaction. NaRbLaNbO5 crystallizes with a tetragonal unit cell [a = 5.839(6) Angstrom, c = 8.313(l) Angstrom] analogous to that of the related compound NaKLaNbO5, while NaRbLaTaO5 indexes to a larger monoclinic unit cell [a = 9.577(2) Angstrom, b = 5.834(l) Angstrom, c = 8.323(2) Angstrom, beta = 93.00(2)]. NaRbLnNbO(5) can be prepared for Ln = Nd, Sm, Eu, Gd, and NaRbLnTaLO(5) can be prepared for Ln = Nd, Sm. Both series of compounds show the expected decrease in unit cell volume as the size of the lanthanide decreases. NaRbLaNbO5 is also amenable to ion exchange, forming Li2-xRbxLaNbO5 upon reaction with molten lithium nitrate. (C) 2004 Elsevier Ltd. All rights reserved.