Films of layered double hydroxides (M/Al-LDHs) with M = Ni, Zn have been fabricated on a porous anodic alumina/aluminum (PAO/Al) substrate via an in situ crystallization technique. The Ni/Al-LDH film has an orientation in which the ab-faces of the platelets are all perpendicular to the substrate whilst the LDH crystallites in the Zn/Al-LDH film are randomly orientated. Furthermore, the interlayer galleries of Ni/Al-LDH contain CO32- anions whereas NO3- anions were found in the interlayer galleries of Zn/Al-LDH. These differences between the Ni/Al-LDH and Zn/Al-LDH films are, at first sight, surprising because the films were fabricated under identical experimental conditions. Two different mechanisms, homogeneous nucleation and the heterogeneous nucleation, have been proposed in order to account for the different growth processes of the Ni/Al-LDH and Zn/Al-LDH films, respectively. The two distinct growth mechanisms can satisfactorily account for the different crystallite orientations and types of interlayer ion in the M/Al-LDH (M = Ni, Zn) films. (C) 2009 Elsevier Ltd. All rights reserved.