Absorption of liquid water by cellulosic materials is a fundamental phenomenon with crucial role in absorbent paper products, printing and packaging materials, as well as in various applications and manufacturing steps of wood products. The imbibition of water in such hierarchical porous materials is difficult to study in high detail. Especially, the absorption and transport of water inside cellulosic fibre walls, which swell and change their nanoscale structure with the introduction of water, remains poorly understood. In this work, we utilized time-resolved X-ray scattering to observe the imbibition of water in paper and wood, investigating in situ the effects of water on the cell or fibre wall nanostructure. Small-angle and wide-angle X-ray scattering (SAXS/WAXS) were measured from wood and paper samples brought into contact with liquid water at the ForMAX beamline of the MAX IV synchrotron. A water reservoir under the sample was slowly filled until the water made contact with the sample, after which water propagation in the sample and the associated nanostructural changes were observed with SAXS and WAXS as function of time and vertical distance from the water surface. An example of one experiment with paper is shown in the attached figure. The scattering data were fitted with models, and the time-dependent changes of the fitting parameters were examined. The results showed similar nanostructural changes with the increasing water content as observed before, including swelling of microfibril bundles and relaxation of lattice strains in the cellulose crystals [1]. The time-dependent behaviour especially in the paper samples revealed that most of the nanostructural changes in the fiber walls took place simultaneously with the propagation of water.