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 mobility within the brush. In contrast to a compact bare brush, for composites a pronounced stretching of the brush after PEM adsorption is observed. The water uptake of the brush is reduced. A PEM on top of a brush is less influenced by the latter one. Swelling gradually reduces the brush/PEM interpenetration, while water accumulates in the interfacial region between the brush and PEM so that these two compartments get increasingly separated with increasing humidity. The swelling process is largely reversible, which demonstrates the potential applicability of these composites as chemical gas or moisture sensors.