Introduction A quantitative, dynamic, and functional understanding of novel natural or synthetically modified peptide species in the native membrane environment requires information about the reconstitutability, positioning, and orientation with respect to the lipid bilayer matrix. Compared to protein complexes, membrane active peptides present relative structural simplicity and are frequently used to validate experimental approaches, techniques, and applications (Bechinger 2001; He et al. 1993) . With the presented study, we are interested in evaluating the biophysical properties of a recently published motif of straightforward designed, aromatic, b-type polypeptides. This includes analysis of peptide positioning, orientation, and changes in bilayer thickness as well as the effect of peptide reconstitution on lateral lipid packing and ordering. Furthermore, we want to introduce a general and applicable X-ray-based approach that provides combined access to significant and quantitative parameters of peptide/lipid interaction on a molecular scale within the physiological membrane state. In this way, structural information, both in the bilayer plane and along the normal direction, can be determined.