Water uptake by thin films composed of the poly(ionic liquid) poly[diallyldimethylammonium bis(trifluoromethanesulfonyl)imide] (PDDATf(2)N) and the ionic liquid N,N-butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr(1.4)Tf(2)N) was studied with a quartz crystal microbalance with dissipation. The data obtained for films with different compositions during the passage of dry and wet N-2 flow through the films were simulated with the Kevin-Voigt viscoelastic model for assessment of the mass of uptake water as well as the viscoelastic parameters. Our results show that the ionic liquid acts as a plasticizer, reducing the rigidity of the film and decreasing the capacity of water uptake. Introduction to a Li salt (LiTf2N) increases the water uptake capacity and also affects both elastic and viscous parameters due to aggregation among the ions from the ionic liquid and Li+. However, due to the preferable interaction of Li+ ions with water molecules, these aggregates are broken when the film is hydrated. In short, the presence of water in such films affects their mechanical properties, which can reflect in their performances as solid state electrolytes and ion-conducting membranes for electrochemical applications.