The structures of two antimicrobial peptides (arenicin Ar-1 and its linear derivative C/S-Ar-1) are studied in different solutions and at the air-water interface using spectroscopic methods such as circular dichroism (CD) and infrared reflection absorption spectroscopy (IRRAS) as well as grazing incidence X-ray diffraction (GIXD) and specular X-ray reflectivity (XR). Both peptides exhibit similar structures in solution. In the buffer used for most of the experiments the main secondary structure elements are 22 % β-turn, 38 % β-sheet and 38 % random coil. The amphiphilic peptides are surface-active and form a Gibbs monolayer at the air-buffer interface. The surface activity is drastically increased by increasing the ionic strength of the subphase. The β-sheet layer is quite stable and can be compressed to higher surface pressures. This adsorption layer is very crystalline. Bragg peaks corresponding to an interstrand distance of 4.78 Å and to an end-to-end distance have been observed. This end-to-end distance can be connected with the observed differences in the layer thickness leading to the assumption that the peptides form a hairpin which is bended depending on the interactions with the counterions.