Polymer additives cause a large increase of the folding strength of paper made of cotton fibres and strengthen less papers made of sulphite or sulphate pulp or linen fibers. The presence of components which cause the formation of fiber-to-fiber bonds in paper (hemicellulose, synthetic polymers) or an increase of the degree to which the pulp is beaten lowers the strengthening effect of impregnation with polymer solutions. The strength of paper containing fillers and adhesives increases insignificantly in the presence of polymers. The best results were observed for paper with fillers. PVA (polyvinyl alcohol) ensures the maximum strengthening effect in comparison with other polymers for the majority of papers.