We studied the swelling and the uptake of water (H(2)O or D(2)O) vapours in polyelectrolyte (PE) multilayer (PEM) samples deposited on solid support (Si wafers) as a function of the isotope nature of the vapour and the charge of the last polymer layer. The samples were prepared with deuterated poly(sodium 4-styrenesulfonate) (dPSS) and poly(allylamine hydrochloride) (PAH). Two types of samples were studied. The sample with a structure Si/PEI/(dPSS/PAH)(6)/dPSS was negatively charged. A positively charged sample was PAH terminated and had the structure Si/PEI/(dPSS/PAH)(6). The film thickness and scattering length density were estimated from neutron reflectometry (NR) experiments and the results were complemented with in-situ QCM measurements. We demonstrate that the swelling of PEM in H(2)O and D(2)O vapours is similar. However, the amount of adsorbed D(2)O is around 10% more than the adsorbed H(2)O. Such isotope effect correlates well with the rough estimation that the isotope effect usually scales with the difference in the mass density of the different isotope forms of the substances. For precise analysis of the NR data we assumed existence of empty voids in the structure of the PEM. These voids might be filled with "condensed" water when the samples are exposed to water vapors. We show that the layers we studied consist of up to 25% of such voids. We showed that the amount of sorbed water depends on the nature of the last layer which builds the PEM thus confirming the "odd-even effect" already shown in the literature.