The reflectivity curves and deduced electron density profiles reveal a stretching of the lipid bilayers upon peptide insertion. This was found for all examined peptide species (2, 5, 7, 8, 9) and is in line with the obtained GID results confirming a positive hydrophobic mismatch. The repeat spacing and the peak-to-peak distance (d pp , compare to Fig. 7) values (Table 2 ) evidence a small thickening of the lipid bilayer up to 1.4 Å , which is more pronounced for the mono-iodinated species 5 and 7. In case of the doublelabeled species 8 and its reference analog 9, this effect is inverted.