Molecular dynamics simulations of tetradecyltrimethylammomium bromide (C(14)TAB) monolayers at the air/water interface were performed at surface areas of 45 and 67 Angstrom(2)/molecule. The computed density profiles normal to the interface for the headgroups and hydrocarbon tails are in good agreement with those derived from neutron reflectivity data. The simulations predict a decrease in the widths of the headgroup and the hydrocarbon chain distributions with increasing surface area/molecule. Analysis of the distributions of the nitrogen atoms within the monolayers by radial and three-dimensional distribution functions showed that the headgroups form a disordered layer. The large widths of the headgroup density profiles seem to be related to an intrinsic roughness of the monolayers and not to a long-wavelength periodic structure. The headgroup radial distribution functions display split first peaks whose inner portion corresponds to about one neighbor at a distance of 6.5 Angstrom. The hydrocarbon chain structure was analyzed by taking into account the headgroup out-of-plane position. This analysis showed that the chains are highly disordered (2-3 gauche conformations/chain) and that their internal structure depends on the vertical position with respect to the interface : A slight difference observed between the chain conformations in the two systems is related to the different width’s of the headgroup distributions. The analysis of the distribution functions of water molecules and bromide counterions with respect to the headgroups indicated that the coordination shell structures around the headgroups are similar in the : two systems. The in-plane diffusion constants for the centers-of-mass of the C(14)TAB molecules were of the same order of magnitude as those in other lipid/water systems.