We sought to construct a monolayer composed of four-a-helical bundles formed by association of di-alpha-helical peptides. Specular X-ray reflectivity showed that dihelices that have been made amphiphilic by attachment of C 16 hydrocarbon chains to their N-termini can be vectorially oriented in Langmuir monolayers at an air-water interface with their helical axes normal to the interface. But off-specular X-ray reflectivity indicated that these dihelices did not associate to form four-helix bundles possibly because they were constrained to be of parallel topology. To achieve four-helix bundles vectorially oriented at the interface, we relaxed this constraint to allow for a 1:1 association of the amphiphilic dihelices with their water-soluble counterparts. Electron density profiles for the monolayers derived from specular X-ray reflectivity demonstrated four-helix bundle formation only when the association between dihelices is directed via designed attractive electrostatic interactions between the polar faces of the amphipathic helices.