This study characterized a panel of monoclonal antibodies (mAbs) raised against cytotoxic α-synuclein oligomers (αSOs) to assess their diagnostic potential for Parkinson's disease, mapping each antibody's affinity and preference for oligomeric, monomeric and fibrillar α-synuclein. Flow Induced Dispersion Analysis (FIDA), on a Fida 1 instrument, was used to measure the hydrodynamic radius of fluorescently labeled α-synuclein species and thereby determine true, in-solution equilibrium binding affinities (KD), revealing that many mAbs bound αSOs with markedly higher affinity than monomers (up to 559-fold) without the surface-immobilization artifacts of ELISA. It is an example of using FIDA to measure antibody binding affinity and target selectivity in solution.