Here we report the results of a low energy electron diffraction (LEED) investigation of monolayer and multilayer films of para-sexiphenyl grown on A1(111) at room and elevated substrate temperatures. For growth at room temperature an ordered monolayer is observed with LEED that indicates the molecules are oriented parallel to the substrate. For further growth up to 80 Angstrom (approx. 16 ML) the LEED pattern remains unaltered indicating a well-ordered film is formed, with the molecules remaining parallel to the substrate. At elevated substrate temperatures the first monolayer is identical to that at room temperature, however, growth on this wetting layer leads to a completely different diffraction pattern that can only be explained by 6P crystallites oriented such that the molecules are near perpendicular to the A1 substrate. The two distinct 2D unit cells that are observed to coexist in this high temperature grown film indicate that the substrate causes a strong deviation from the bulk crystal structure of sexiphenyl. (C) 2003 Elsevier Science B.V. All rights reserved.