Small-angle X-ray scattering (SAXS) is one of the major tools for the study of model membranes, but interpretation of the scattering data remains non-trivial. Current approaches allow the extraction of some structural parameters and the electron density profile of lipid bilayers. Here it is demonstrated that parametric modelling can be employed to determine the polydispersity of spherical or ellipsoidal vesicles and describe the electron density profile across the lipid bilayer. This approach is implemented in the computer program BILMIX. BILMIX delivers a description of the electron density of a lipid bilayer from SAXS data and simultaneously generates the corresponding size distribution of the unilamellar lipid vesicles.