Absorption of water in cationic polyelectrolyte thin films is investigated by measuring mass uptake and mechanical stress using a quartz crystal microbalance and scanning optical laser apparatus, respectively. Thin layers of poly(vinylbenzyl chloride) and diazabicyclo[2.2.2]octane are spin-cast onto quartz crystal sensors or cover glasses. Films are subsequently cured and reacted by nucleophilic substitution with trimethylamine, triethylamine, tripropylamine, or tributylamine to give an immobilized poly-(vinylbenzyltrialkylammonium chloride) matrix. Water absorption in these films depends strongly on both the amine modifier and counterion present in the matrix. Mass uptake ranges from 5.7% for a tributylamine-modified film in perchlorate form to 52% for a trimethylamine-modified film in fluoride form. This water uptake results in a maximum relative compressive biaxial stress of -33 and -113 MPa for these two films, respectively. Mass uptake and biaxial stress data give evidence of yielding upon hydration and dehydration. The yielding is attributed to elasto-viscoplastic deformation and influenced by a depression of the glass transition temperature via plasticizing. The Young's modulus (Y-f) of the unmodified film is measured from biaxial stress and ellipsometric thickness to be 3.9 GPa, consistent with structurally related polystyrene and slightly lower than Y-f = 5.0 GPa as measured acoustically. These results support the prevailing theory relating selectivity and diffusion in perchlorate-selective anion-exchange resins to hydrophilicity and water content and also suggest the mechanical properties of hydrophobic polycations may be effectively controlled through judicious selection of fixed ion and counterion.