The combination of polyelectrolyte multilayers (PEMs) and end-grafted brushes represents a valuable approach to the design of complex organic composite materials with tailored responsive properties. This article addresses the correlation between the swelling properties of composites and their internal structure. Here, we present composites of end-grafted 2-(methacryloyloxy)ethyl-trimethylammonium chloride (PMETAC) brushes covered with poly(sodium styrenesulfonate) (PSS) and poly(diallyldimethylammonium) chloride (PDADMAC) PEMs and characterize them under various relative humidities. Ellipsometry and neutron reflectometry are carried out to monitor the swelling behavior and the internal structure, respectively, of the composites, giving evidence of a significant mutual influence of the two components. The reflectivity data reveal a deep penetration of the PEMs’ polyelectrolyte chains into the underlying brush but also a significant humidity dependence of their spatial distribution, indicating considerable ...