Ellipsometry was used to characterise the adsorption of polyelectrolytes onto oppositely charged polyelectrolyte brushes and the formation of polyelectrolyte multilayers, enabling the exploration of the role of brush thickness, pH and the presence of additional electrolytes . However, neutron reflectometry allows to gain further insight into the morphology of the resulting complexes and identify molecular diffusion within resulting assemblies . The high z-resolution of neutron reflectometry enabled the determination of the morphology of polyelectrolyte-polyelectrolyte brush complexes and the identification of the localisation of infiltrating polyelectrolytes .