Water interactions of ultra-thin films of wood-derived polysaccharides were investigated by using surface sensitive methods, Quartz Crystal Microbalance with Dissipation (QCM-D) and Atomic Force Microscopy (AFM). This approach allows systematic molecular level detection and reveals the information of biobased materials’ inherent behaviour with nanosensitivity. The influence of structural features of cellulose films i.e. crystallinity, surface roughness and porosity on water interactions were clarified. Cellulose films were prepared using spin-coating and Langmuir-Schaefer deposition to obtain thin films of equal thickness, identical cellulose origin, simultaneously with different supramolecular structure. The uptake/release of water molecules and swelling were followed using QCM-D, and the structural features of the films were evaluated by AFM. More crystalline cellulose film possessed nanoporosity and as a consequence higher accessible surface area (more binding sites for water) and thus, it was capable to bind more water molecules in humid air and when immersed in water when compared to amorphous cellulose film. Due to the ordered structure, more crystalline cellulose film remained rigid and elastic although the water binding ability was more pronounced compared to amorphous film. Lower amount of bound water induced softening of the amorphous cellulose film and elastic layer became viscoelastic at high humidity. Finally, cellulose thin films were modified by adsorbing a layer of 1-butyloxy-2-hydroxypropyl xylan, and the effect on moisture uptake was investigated. It was found that the supramolecular structure of the cellulose substrate has effect not only on the adsorbed amount of xylan derivative but also on the water interactions of the material.