This study shows that Flow Induced Dispersion Analysis (FIDA) can characterize how bispecific antibodies (bsAbs) bind two different antigens at once, entirely in solution and without attaching molecules to a surface. Using a Fida 1 instrument, the authors measured the apparent hydrodynamic radius of a fluorescently labeled antigen as it formed larger complexes with antibodies and a second, unlabeled antigen. Stepwise size increases confirmed simultaneous bispecific binding, while titrations yielded individual equilibrium dissociation constants for each binding event, revealing how molecular format and fusion site affect affinity. The method worked across diverse bsAb architectures and valencies with low sample consumption and high matrix tolerance, requiring little optimization. The authors conclude that FIDA is a versatile tool for selecting formats and characterizing complex bi- and multispecific protein therapeutics through drug development and biomanufacturing.