A process is disclosed for preserving paper involving the following steps:(a) a plurality of paper layers in contact with one another at least in regions is simultaneously brought into contact with a preservative fluid containing polycondensates based on one or more hydrolytic polycondensation-capable compounds of elements M of main groups III-V and sub-groups II-IV of the Periodic Table; in said polycondensates, at least one organic group (G) with at least two carbon atoms and for each carbon atom at least one atom selected from hydrogen or fluorine and without polar substituents is bound, either directly via one of the carbon atoms or via a connecting group A, to at least 0.1% of the central atoms M; and(b) the paper thus treated with the preservative fluid is dried and/or hardened.