A new series of transition-metal oxyhalides (MCl)LaNb2O7 (M = Cr, Mn, Fe, Cc) have been prepared by a simple topochemical route. Layered perovskite hosts (ALaNb(2)O(7), A = Li, Na, K or Rb) were reacted with the corresponding anhydrous metal halides under mild reaction conditions (<400 degreesC). The compounds were examined by X-ray powder diffraction; the series appears to be isostructural with (CuCl)LaNb2O7, and the layer spacings, with the exception of M = Co, follow the trend expected from transition-metal cationic radii. Thermal analysis with differential scanning calorimetry (DSC) shows the materials to be metastable where all four compounds decompose exothermically above 690 degreesC.