In this paper, we propose a new kinetic model that describes the depolymerization of pure cotton cellulose, applicable to ageing experiments in sealed vessels. By recording the pH of the salt solutions used for controlling the relative humidity inside the vessels, we demonstrate that volatile acidic species are produced during the ageing of pure cellulose. Based on the proposition that the acids produced must have an autocatalytic effect on the depolymerization of cellulose, we theoretically developed equations describing the time dependence of the percentage of the broken glycosidic bonds, the folding endurance, tensile strength, tensile energy absorption, stretch at break and the L* and b* coordinates of the CIEL*a*b* color system. The experimentally determined values of these pure cellulose paper properties were fit into the model equations by use of a computer statistical package. The very high values of the regression coefficients indicate that the model equations work quite well. The experimental data and the model equations indicate that for long ageing times (up to 150 days at 80 °C), the deterioration of the sample properties accelerates and finally brings about an abrupt breakdown of the samples. Under the light of these findings, we believe that the useful life of the historical paper stored in archives and libraries is less than predicted by the existing models, and steps for its preservation should be taken immediately. As an additional preservation measure, we propose the ventilation of the storage areas so that the volatile acidic products of paper deterioration that accelerate its ageing are removed.