There are well-established X-ray based methods for elucidating the structure of lipid bilayers. Here we measured the thicknesses of the GDGT membranes from small-angle X-ray scattering (SAXS) on unilamellar vesicles in solution. In this method, only diffuse scattering is obtained from uncorrelated membranes and therefore no Bragg peaks are observed. The observed scattering intensity profiles result from the difference in scattering length densities between the lipids and the buffer. In a typical experiment, we obtained scattering intensity profiles from GDGT unilamellar vesicles characterized by three broad lobes (Figure ). The profiles were fitted to a well-established model based on a simulated annealing algorithm that has been previously used to compute asymmetric bilayer electron density profiles (EDP); all fitting parameters are provided in Table . A typical EDP is characterized by two peaks corresponding to the electron-rich lipid headgroup regions separated by a trough corresponding to the electron-deficient hydrocarbon region (Figure ). Therefore, the head-to-head distances in a lipid membrane can be readily measured from the peak-to-peak distances of the EDPs which ranged between 38-40 Å for the GDGT lipids (Figure ).