refolding at/in membranes, we used a characteristic feature of SEIRAS, i.e., the surface-selection rule. Due to the plasmonic surface-enhancement mechanism of SEIRA, the electromagnetic nearfield is polarized parallel to the surface normal (i.e., perpendicular to the Au surface and membrane plane; here the z-axis in Figure ). As such, only vibrations with transition dipole moment contributions along the z-direction are excited and contribute to the spectra. In our previous work we exploited this property of SEIRA successfully to quantify the orientation of α-helices in antimicrobial peptides and viral ion channels with an very good accuracy of around ±5°: we used the orthogonality of the transition dipole moment vectors of the amide I and amide II modes; that is, we used their characteristic orientation-dependent intensities in SEIRA spectra. In the case of the β-sheets, we have three orientation dependent marker bands, i.e. the two amide I modes at 1694 and 1634 cm , and the amide II mode at 1560 cm -1 . These three observables do not only enable us to calculate the orientation of β-sheets at the membrane, but also to determine if a certain spectrum is more compatible with antiparallel and/or parallel βsheets. It should be noted that the intensity ratio between 1694 and 1634 cm -1 (the so-called β-index) has been used previously as metric for the antiparallel vs. parallel β-sheet ratio. As such, we are extending this strategy by including the amide II intensity.