Some empirical and semiempirical equations were applied to folding endurance and reflectance data from kraft paper samples aged at 70-100 degrees C. Better fits to the data were obtained with a plot of the logarithm of the property vs. The square root of ageing time rather than the ageing time itself. Schemes with consecutive, competitive, or reversible reactions were considered, thus deriving at equations that could describe the change of properties of paper with time. With diagrams and tables.