Microfibrillated cellulose (MFC) has unique properties which can lead to interesting new paper applications. Some applications strongly rely on defined pore structure, like filter paper for water purification or separator paper in batteries. The pore structure modification of a paper sheet through MFC is investigated, the combination of MFC and polyelectrolytes (PEs) enables a pore dimension reduction of an existing softwood softwood base layer. In both handsheet and pilot paper machine trials this effect was accomplished and reproducible. It was shown that MFC grammage, MFC consistency and PE type are significant factors that affect the pore dimension.