The structural organization of Langmuir monolayers of the alpha-helical peptide alamethicin, which forms voltage-gated ion channels in lipid membranes, is studied by means of synchrotron X-ray diffraction, surface potential technique, surface pressure/area isotherms, and atomic force microscopy (AFM). Alamethicin adsorbs and adopts a parallel orientation of its helix axis at interfaces, forms 2D crystalline aggregates, and phase-separates from lipids. The structural results obtained correlate with the novel AFM images of alamethicin helices. The effects of hydrogen-bond promoters, pH, salt content of the aqueous subphase, and lipid-peptide interactions were analyzed. The modification of the intrapeptide hydrogen-bond strength, by H2O2 hydrogen-bond promoter, affects the helix structural parameters and the density of the crystal structure.