We would like to shed some light on the electrostatic assembly mechanism and morphological signature(s) at the nm scale in pseudo-equilibrium conditions of model oppositely charged polyelectrolytes by combining the unique potential of SANS with a dialysis set-up allowing a fine tuning of the magnitude of the interaction and the reaction time through a slow desalting transition pathway. The monitoring of the inner structure of two well-known (PDADMAC/PANa and PEI/PSSNa) systems will be investigate as a function of the molar and the molecular weight charge ratios and the ionic strength difference.