The effect of a floating stearate monolayer on the formation of Mg-Al-hydrotalcite (Mg-Al-LDH) has been studied. A subphase was an aqueous solution of Mg(NO3)(2).6H(2)O (1.6 x 10(-3) M) and AI(NO3)(3).6H(2)O (5.3 x 10(-4) M) adjusted at pH = 10.5 by 1.0 M NaOH. Templating effects were studied in the following two ways. First, we spread a chloroform solution of stearic acid onto the above subphase and waited 6 h at zero surface pressure (0.51 nm(2) molecule(-1)). After the surface was compressed to 20 mN m(-1) (0.26 nm(2) molecule(-1)), the floating film was deposited onto mica as a Z-type film (method 1). Second, we spread the same chloroform solution onto the subphase and started to compress the surface to the molecular area (0.40-0.20 nm(2) molecule(-1)) after 30 min. The floating film was maintained at the constant surface pressure for 6 h. Thereafter, it was deposited onto mica as a Z-type film (method 2). From the atomic force microscope (AFM) images of the films, it was concluded that the largest thin crystals (ca. 3 x 10 mum) with the thickness of 10 1 mu were obtained according to method 2 when the deposition was done at 0.36 nm(2) molecule 1 (0.78 mN m(-1)). This optimum molecular area was close to the area occupied by one negative charge for 3:1 Mg-Al-LDH (0.34 nm(2)). From the X-ray diffraction measurements and elemental analyses, the deposited films prepared by both methods were suggested to be Mg-Al-CO32--LDH. As a comparison, a 0.1 mL portion of the subphase solution, which had been aged for 6 h, was cast onto mica and dried under the air. The AFM image of such a cast sample showed noncrystalline aggregates of small particles with the diameter of 0.2-3 mum. These results indicated that a stearate monolayer acted as a template for the crystallization of Mg-Al-LDH at an air-water interface.