Polyion complex (PIC) micelles formed from the electrostatic interaction between oppositely charged polymers have been studied for their promising applications in the biomedical field as drug carriers or vectors for gene delivery. In spite of their asset of possible high drug loading, their formation process remains poorly studied. In this work, we investigate the properties of a series of PICs based on poly(ethylene oxide-b-acrylic acid) (PEO–PAA)/dendrigraft poly(L-lysine) (DGL3), using PEO–PAA with different compositions and average molecular weights. For each PEO–PAA/DGL3 pair, the complexes were characterized as a function of the ratios between acid and amine moieties combining different techniques: dynamic light scattering (DLS), flow field-flow fractionation (FlFFF), small-angle X-ray scattering (SAXS), and relaxometry. The coupling of batch techniques, i.e., DLS, SAXS, and relaxometry, together with a soft separation technique like FlFFF enabled a finer analysis to elucidate subtle details of the association process and of the polydispersity of the complexes. We show that the formation of PICs is more complex than previously described. In particular, we demonstrate that PICs with stoichiometry 1:1 may form at low ratios provided that the acidic block is long enough to neutralize the cationic dendrigraft with few polymer chains. Moreover, in such conditions, PICs with stoichiometry 1:1 often coexist with free dendritic polymers and other associated complex species.Read less <