In a single procedure, Taylor dispersion analysis (TDA) was used for the size characterization of polyplexes and the quantification of free polycation contained in excess within the polyplex sample. TDA analysis was carried out in frontal mode for a better sensitivity of detection. The proof of concept was established using a model polyplex generated from the mixture of linear polylysine (DP 20) and DNA from salmon testes at nitrogen to phosphate (N/P) ratio of 12. Polyplex hydrodynamic radius was compared to the values obtained by dynamic light scattering measurements. TDA was found to give access to the weight-average hydrodynamic radius, while DLS basically gives an intensity-average (harmonic z-average) value. The method was next applied to the study of various polyplexes issued from polylysines of various DP (50, 100) and different topologies (dendrigraft polylysines of generation 2 and 3). This new methodology should greatly contribute to the physicochemical characterization of polyplexes used for gene transfection.