The combination of neutron reflectometry and ellipsometry data presented clearly confirms the deep infiltration of RNA oligonucleotides within dense polycationic brushes, in agreement with the previously reported high adsorption levels observed for these brushes via SPR. In contrast to other strong polyelectrolyte interactions previously studied, surface complexation and hindered infiltration was not observed, even in the case of quaternised PMETAI brushes. The associated structure of RNA-brush complexes is therefore proposed to facilitate the retention of oligonucleotides and prevent competitive binding events with other macromolecules present in the surround environment during delivery or within the cytosol, upon uptake. Interestingly, for all PDMAEMA brushes studied, the lack of overall change in brush thickness is at odds with the increase observed via in situ ellipsometry. Therefore it may be proposed that the increased ellipsometric thickness measured reflects an increase in complex density and associated refractive index. However, ellipsometry data do not allow distinguishing the relative roles of refractive index and complex thickness and neutron reflectometry therefore enables greater insight into the architecture of RNA-brush complexes. In this respect, results imply that the combination of grafting density, thickness and chemical structure of polymer brushes regulate the morphology of RNA complexes. However, the changes in scattering length densities observed did not allow to completely resolve the architecture of complexes generated. Indeed, whilst SLDs were found to increase in all brushes studied and, although these changes were observed relatively deep within corresponding structures, the relatively high SLD of the medium and that predicted for oligonucleotides did not allow to resolve whether changes in SLD were reflecting an increase in macromolecular density or hydration. Further studies and refinement of contrast strategies are therefore required to fully elucidate the detailed architecture of RNA-brush complexes.