where ρ salt and ρ T ris are the SLDs of the specific salt solution outside the vesicles and of salt-free Tris buffer, respectively. For the vesicle references in Tris, ρ salt = ρ T ris and f buf f is not used. When the vesicles are mixed with salt, ρ salt is the SLD of the final salt concentration and f buf f is a fitting parameter. Hence, f buf f = 0 corresponds to no salt in the vesicle interior, while f buf f = 1 corresponds complete equilibration, with same concentration on both sides of the membrane. This parameter allows us to quantify differences in the interior and exterior salt concentrations and, combined with the time resolved SAXS data, to track the transport of salt across the membrane. The notation of f buf f (t) is used to highlight that this parameter is time-dependent. It is strongly emphasized that the buffer SLDs for Tris with 0, 0.15, 0.30 and 0.60 M NaCl, respectively, were independently calculated from the specific densities (measured with Anton Paar DMA 5001 Density Meter) at both 20 and 37 °C. These values are tabulated in the SI (Table ), and are kept fixed in the fitting procedure.