Oriented crystalline thin films of alpha,omega-tetracosanedioic acid metal salts form in situ at the air-liquid interface, following the simple procedure described here. It is shown that the Cd2+ and Pb2+ metal ions have a dramatic influence on the packing of the diacid molecules, which self-assemble as multilayer domains approximately 50 Angstrom thick, with the molecules and bound ions perfectly ordered laterally over distances up to 1000 Angstrom. These oriented crystallites preserve their integrity upon transfer to a mica support and can be used to prepare quantum dots (see following communication).