The aim of this work is to investigate the influence of fiber fibrillation and fines on the pore structure of well-defined regenerated fiber sheets as well as the water flow through the sheet. For this purpose, sheets were produced with refined, fibrillated fibers only, with unfibrillated fibers and fines, as well as with fibrillated fibers and fines. Next, the samples were analyzed by brightfield and fluorescence microscopy, mercury porosimetry, and an ascending test. Both the fibrils and the added fines reach into the pores between the fibers or are deposited there. As a result, pore size decreases and capillary flow slows down. The two effects overlap when the fiber surface is fibrillated and fines are present. Sheets with thicker fibers form a pore structure with larger pores in between the fibers. However, such a change in pore size has no significant influence on the flow of water through the sheet in the performed ascending tests. It is shown that a statistical model with the parameters fibrillation and fines content can be used to describe the ascending rate nearly as well as the Lucas–Washburn equation. Consequently, the equation could be improved by the addition of further fiber and sheet properties.