A new metastable double-layered perovskite, Rb0.04Na0.08Ca0.44LaNb2O7, has been synthesized by a two-step ion exchange route. Initially, RbLaNb2O7 is reacted to produce the known metastable phase NaLaNb2O7; then this is exchanged with calcium to yield the title compound. The product has neither the RbLaNb2O7 nor NaLaNb2O7 structure types, but converts to a superstructure of KLaNb2O7 with an orthorhombic unit cell: a = 3.885(1), b = 7.725(2), and c = 20.884(4) Angstrom. Direct calcium exchange of RbLaNb2O7 results in poorly crystalline products with only 31% exchange, while exchange with KLaNb2O7 shows little evidence of reaction. The importance of size matching in divalent-for-monovalent cation exchange is discussed.