Many biomaterials exhibit pronounced swelling and consequently pronounced porous structure when exposed to water. Characterization and tuning of the porosity are important for the fundamental understanding of the behaviour of the biomaterials as well as for many of their applications, both traditional and novel. Here, the porous structure of cellulosic fibres (chemical wood pulp) was analysed in the wet state by differential scanning calorimetry (DSC) with and without adsorbed cationic polyelectrolytes. The polyelectrolytes were low molecular weight (M_w) high charge polybrene (hexadimethrine bromide) and two high M_w high charge PDADMACs (poly(diallyldimethyl ammonium chlorides)) with well-defined M_w distributions. The porosity changes upon adsorption of cationic polyelectrolytes in the wet-state were followed and the pore analysis gave insights into the distribution of the pores in the wet-state and into the changes of the porous structure in the surface as well as within the whole cell wall. By utilizing the well characterized polyelectrolytes we were able to detect subtle changes in the micropores of cellulosic fibres due to the adsorbed polyelectrolytes. The polymers did not affect the pore volumes in the 2.5–17 nm region upon drying, an important finding considering the porosity. Overall, the cationic polyelectrolytes decreased the pore volume by reducing the osmotic pressure. In addition, the results were compared to a physical state change, i.e., drying and rewetting, to observe and compare the wetting hysteresis of the aforementioned fibres.