The odd–even effect, i.e., the influence of the outermost layer of polyelectrolyte multilayers (PEMs) on their swelling behavior, is investigated. For that purpose poly­(styrene sodium sulfonate) (PSS)/poly­(diallyl-dimethylammonium chloride) (PDADMAC) polyelectrolyte multilayers are studied in air with 1% relative humidity (RH), 30% RH, 95% RH, and in liquid water by ellipsometry, atomic force microscopy (AFM), and X-ray reflectometry (XRR). Since the total amount of water uptake in swollen PEMs is divided into two fractions, the void water and the swelling water, a correct evaluation of the odd–even effect is only possible if both fractions are examined separately. In order to allow measuring samples over a larger thickness regime the investigation of a larger amount of samples is required. Therefore, the concept of separating void water from swelling water using neutron reflectometry is for the first time transferred to ellipsometry. The subsequent analysis of swelling water, void water, and roughness revealed the existence of two types of odd–even effects: an odd–even effect which addresses only the surface of the PEM (surface-odd–even effect) and an odd–even effect which addresses also the bulk of the PEM (bulk-odd–even effect). The appearance of both effects is dependent on the environment; the surface-odd–even effect is only detectable in humid air while the bulk-odd–even effect is only detectable in liquid water. The bulk-odd–even effect is related to the osmotic pressure between the PEM and the surrounding water. A correlation between the amount of void water and both odd–even effects is not found. The amount of void water is independent of the terminated layer and the thickness of PEMs.