Liquid ammonia was used to treat films of nanofibrillated cellulose (NFC), trimethylammonium-modified NFC (TMA-NFC), and their composites with vermiculite. Crystal structure, mechanical properties, water vapor permeation and water vapor adsorption of the resulting materials were investigated. Upon treatment, the crystal structure of (TMA-)NFC both in presence and absence of vermiculite changed from cellulose I to III. With the exception of TMA-NFC/vermiculite composites, pronounced effects on the addressed mechanical properties arose after exposure of the materials to ammonia. Furthermore, treatment of composite films with ammonia led to a distinct decrease in water vapor permeation. Remarkably, TMA-NFC/vermiculite composites films show the best water vapor barrier properties, highest tensile strength and highest elastic modulus after treatment with liquid ammonia. This is regarded to be at least partially a consequence of electrostatic attraction between the positively charged ammonium groups in TMA-NFC and the anionic silicate layers of vermiculite. (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013