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SUMMARY:Recent findings have shown that it is possible to use the Layer-by-Layer technique to create nanofibrillated cellulose / polyethyleneimine interference films whose colour change with relative humidity. This study uses different optical models to describe spectral ellipsometry measurements data of interference films and how the film properties alter in dry and humid environments. The results indicate that water condensation initially is filling the surface pores within seconds whereas relaxation of the film to adjust to the added water is a slower process that reaches a steady state after 20 min. The maximum swelling ratio of the LbL films is almost independent of the number of layers within the film, but decreases considerably by crosslinking via heat treatment. The films show a distinct birefringence with optical axis perpendicular to the surface. Analysis of the moisture response with different optical models indicates that the films swell uniformly in the thickness direction with no separate water film on top. The results provide important understanding for the design of NFC based LbL films for visual moisture sensors and interactive security paper. Since the introduction in the late 1980s, the Layer-byLayer (LbL) technique (Decher et al. 1991, Decher 1997 has initiated a large interest in the scientific community. Using the LbL technique, it is possible to construct welldefined nanometer thin layers of oppositely charged polyelectrolytes and/or colloids at any solid-liquid interface. ADDRESSES OF THE AUTHORSDuring the last decade, there has been a large focus on using NanoFibrillated Cellulose (NFC) (Klemm et al. 2011) in new materials. NFC emanates from the cellulose fibrils inside the wood fibre wall and the fibrils are liberated in different ways using both chemical treatment and high pressure homogenization (Klemm et al. 2011;Eichhorn et al. 2010) to separate the fibrils from each other and to stabilize them in aqueous dispersion. Depending on the preparation conditions, it is also possible to create negatively as well as positively charged NFCs with different charge densities (Klemm et al. 2011) on the nanomaterial surfaces. This naturally affects the stability of the dispersions, their self-associative properties and their interactions with other nanomaterials.It has been shown that the build-up of the LbL structure is so regular that the deposition of each NFC layer ) creates a colour change after a few bilayers of polyethyleneimine (PEI) and NFC on a silicon wafer. This opened totally new possibilities to create interactive products , such as for example moisture sensors, since the layers are sensitive to moisture and will change colour depending on how much water that is adsorbed into the LbL structure. Another very interesting application is in security paper where LbL structures are first formed and then transferred to paper by for example microcontact printing. However, in all these applications it is necessary to determine how moisture interacts with the NFCcontaining LbL films. Previou...