The aim of this work was to understand the particle level swelling and pore structure of microfibrillated cellulose (MFC). For this purpose, a new variant of the solute exclusion test was constructed which takes into account the adsorption of dextran onto the cellulosic material and the elastic response of the fiber material to external osmotic pressure. With the new method, two important properties, fiber saturation point at zero external osmotic pressure (FSP0) and isotropic elastic modulus could be obtained. The particle level swelling for MFC was found to be 1.6 ml/g which is about the same as the swelling of the parent pulp fibers. The MFC swelling was con¿ rmed with thermoporosimetry which yielded further insights into the development of pore structure and surface area when the fiber cell wall is defibrillated.