The paper addresses the swelling of polyelectrolyte multilayers (PEMs) at varying humidity. In particular, a new model will be presented, which takes the gradual filling of voids into account. Absorption of water results in PEM swelling. This absorbed water can be distinguished into swelling and void water. Swelling water leads to an increase in thickness and a change of the optical properties of PEMs, while the void water results only in a change of the optical properties. In former studies, neutron reflectometry was used to distinguish between swelling and void water. However, as we show in this study, it is possible to resolve the two different kinds of water in PEMs by ellipsometry, a much simpler tool. The present study evaluates and interprets the refractive index of polystyrenesulfonate/polydiallyldimethylammonium chloride (PSS/PDADMAC) PEMs. Both the swelling behavior and the refractive index change as a function of relative humidity and were found to be independent of the layer number. The void model and the extended void model were used to describe the data. The void model allows fitting the experimentally determined refractive index at humidity beyond 20% RH but fails for humidity lower than 20% RH. Therefore, we modified the existing model in order to account for air-water exchange. The extended void model assumes a gradual air-water exchange at low h and describes the refractive index over the entire humidity range in a precise way. Up to 30% RH, air and water coexist. Above this threshold, the voids are completely filled with water and this threshold does not change either with layer number or with the outermost layer. Furthermore, this model allows the determination of the volume fraction of the voids (0.05 +/- 0.01) and the refractive index of the pure polymer matter (1.592 +/- 0.002).