In general, the electron contrast variation resulted in changes in the headgroup densities, which were too pronounced to be explained by the contrast variation itself and therefore indicate systematic errors. These probably stem from misalignment or drift when changing the photon energies. Thus, the two approaches of contrast variation (in-house and synchrotron) are not in quantitative agreement. For the in-house measurement, the effect of a decrease in electron density contrast of the side minima compared to the global minimum is observed in most peptide/lipid systems. The decrease can be explained by a beginning lipid disorder that evens the density profile by increased density in the water layer and/or an enlarged area per headgroup ; this effect may be more pronounced for a peptide carrying a bulky iodine label. However, we also cannot rule out systematic errors for the in-house measurements, and beyond the thickening effect, which appears to be robust, the small differences between curves with and without labels may impede an unambiguous localization of the iodine position. Finally, we must be cautious since iodine-labeling itself may influence peptide/lipid interactions. However, there is no obvious reason why attachment of the iodine labels at different positions should cause different peptide positioning, especially as the iodine labels have been shown to have only minor effects on the peptide secondary structure (Schneggenburger et al. 2009 ).