Very interestingly, with prior AMP incubation the curves for the increasing salt concentrations can be described almost entirely by simply setting the internal salt concentration equal to the external. In other words, the electron density of the buffer, which is dependent on the salt concentration, is equal on both sides of the membrane. This corresponds to complete salt equilibration across the membrane, which would negate the osmotic pressure, and explains the lack of observed deformation. In this way SAXS provides a very convenient, and importantly label-free, way of probing the salt concentration in the vesicle interior. The model analysis further shows that, when peptide is added, the vesicles remain intact with only minor changes in the bilayer structure. In excellent agreement with several previous studies , the peptides insert primarily in the outer headgroup shell, with 85 and 90 % for indolicidin and LL-37 respectively. The remaining 15 and 10 % respectively, insert in the central hydrocarbon shell, with no peptide partitioning to the inner headgroup. Additionally, neither the partitioning of the peptide nor bilayer structures change with increasing salt concentration. The full set of fitting parameters and their values are tabulated in table S6 in the SI.