Cellulosic materials constitute the basis for a variety of products such as several kinds of papers, medical and hygiene articles, as well as building materials. One of the most important cellulose products is paper, with uses ranging from data storage to packaging applications. Even though papermaking is practiced and continuously improved for several thousand years, the detailed mechanisms constituting a bond between two fibers are still elusive. In fact, some mechanisms – ranging from hydrogen bonds to mechanical interlocking – have been proposed, however, it is not clear to which extent these mechanisms contribute or if all of them have been identified yet. But exactly this information is of crucial importance when strength of paper is to be improved, without increasing the material input. Hydrogen bonds and similar interactions exhibit a maximum effect when the area of the fiber-fiber bond in molecular contact is as high as possible. This would be the case when the surfaces are smooth, soft, and compliant. To assess the structure and mechanical properties of surfaces, atomic force microscopy (AFM) and AFM based nanoindentation (AFM-NI) was employed in this work. A developed measurement setup provided a possibility to control relative humidity and thus to study the influence of water on the materials under investigation. Also, measurements of fully swollen fibers in water were facilitated. This corresponds to the state in which bonds initially form during papermaking. Since paper fibers are a complex and inhomogeneous system, the characterization started with amorphous cellulose films and viscose fibers as model systems. These chemically and structural homogeneous materials permit to focus only on single mechanisms. Topographical AFM investigations of cellulose films – before and after a bond had been formed between them – revealed that the initial roughness of 30 nm decreased to one third after the rupture of the bond. It was further observed that the surfaces were homogeneous which confirms that smooth surfaces are able to form a large area in molecular contact. All materials' mechanical properties were determined by AFM-NI as reduced modulus – a measure for the Young's modulus – and hardness under controlled humidity as well as in water. Viscose and pulp fibers both exhibited a reduced modulus in water of about 0.05 GPa. For viscose fibers, this corresponds to a reduction by a factor of almost 200 compared to the dried state. Pulp fibers, on the other hand, are more compliant in dry conditions and, therefore, the reduced modulus in water decreased only by a factor of 100 to 150. The surface hardness of viscose fibers in water is approximately 14 MPa which is a decrease by a factor of 30 compared to the fully dried state. Pulp fibers, on the other hand, exhibited a hardness of 7 MPa in water, which corresponds to a decrease by a factor of 50 to 100, with respect to the dried state. Both materials exhibited an approximately linear relation between hardness and modulus with relative humidity. A deviation from the linear behavior was observed only at high humidities of more than 80%. The study has further proven that AFM-NI is an appropriate tool to investigate cellulose based materials under various relative humidities as well as fully swollen materials in water.