We performed a series of molecular dynamics simulations of an N,N'-di-n-butyl-3,4:9, 10-perylene tetracarboxydiiimide (BuPTCD) Langmuir film. The film was studied at three surface areas (0.38, 0.40. and 0.45 nm(2) molecule(-1)) using model systems consisting of one BuPTCD monolayer on each side of a 6.0-nin-thick water slab. On the basis of geometrical reasoning and surface pressure results, it was found that perylene molecules seem to be oriented mainly head-on toward the water surface. On the other hand. the average orientation of the perylene tetracarboxidiiinide moiety during simulations indicated that molecular alignment is not simple, changing from tilted face-on to tilted head-on during compression. The change in orientation of BuPTCD molecules was accompanied by the thickening of the film as the area decreased. Films also became more orderly at small surface areas, mostly at the rigid perylene core. BuPTCD molecules are not evenly hydrated. with very few solvent molecules hydrating the hydrophobic tails, whereas a considerable number of water molecules were hydrogen bonded to diimide oxygen atoms. Besides, the residence times of water molecules around perylene atomic sites increased from the terminal methyl groups to the diimide oxygen atoms. Finally, the electric potential profile across the rnonolayer was found to depend on the area occupied by each molecule, indicating that properties of the films may vary considerably with the area occupied by each molecule. Altogether, the molecular description arising from our computer simulations suggests which structural patterns may be found in these films depending on the surface density of perylene molecules.