The impact of aqueous treatments on the strength properties of Whatman filter paper and five historic papers were investigated. The results showed the statistically significant decrease of tensile strength of Whatman paper and four of the historic papers due to the treatments. They also showed mixed trends concerning folding endurance and tensile energy absorption. Possible explanations for the strength decrease are proposed and long-term implications are discussed on the basis of different strength decrease scenarios. It is suggested that aqueous treatments should be applied only when necessary and that they should be followed by consolidation. The importance of strength and especially tensile tests is emphasized, as they can register changes that cannot be detected by chemical methods. It is suggested that the lack of a preconditioning step can result in misleading conclusions concerning the folding endurance of treated samples.