This study dissected how the intrinsically disordered protein α-synuclein binds and inserts into lipid membranes, a process central to its physiological role and to Parkinson's disease. Using NMR, SAXS, circular dichroism, neutron reflectometry, X-ray diffraction and cell imaging, the authors showed membrane binding relies on avidity within an N-terminal anchor. Flow Induced Dispersion Analysis (FIDA) was used to measure the diffusion coefficient and hydrodynamic radius of fluorescently labeled α-synuclein as it bound anionic POPG vesicles, generating binding isotherms to quantify membrane-binding affinity (Kd) and rank α-synuclein variants by competition. It is an example of using FIDA to measure protein–membrane binding affinity in solution.