Aqueous dispersions of C6-carboxylated cellulose nanofibrils with sodium counterions (CNF-COONa) and CNFs with tetraethylammonium counterions (CNF-COONEt4) were mixed at various weight ratios. Transparent, flexible CNF-COONa/NEt4 films were prepared by casting and drying aqueous mixtures with various Na/NEt(4 )molar ratios as dual counterion systems. The film density, Young's modulus, and tensile strength decreased linearly with increasing molar ratio of the bulky NEt(4 )counterion. The film hydrophilicity was controlled by varying the Na/NEt(4 )molar ratio. The oxygen and water vapor permeabilities also increased with increasing molar ratio of bulky NEt(4 )counterions. The mechanical, hydrophilic, and gas-barrier properties were tuned by controlling the Na/NEt4 molar ratios of CNF films containing dual counterions. The results of model experiments using tetra(n-butyl)ammonium hydroxide indicated that the Na and quaternary alkylammonium counterions were homogeneously distributed among the carboxylated CNF elements in both dispersions and cast-dried films of the dual counterion systems.