Degradation of paper varies according to the concentration of potassium aluminum sulfate (PAS) in the animal glue sizing solution (* 2dosa*1). Japanese handmade paper samples applied with a sizing solution of PAS (0.5% and 5%) or aluminum sulfate (AS) (0.5% and 5%) and 1% animal glue in water were artificially aged at 80°C and 80% RH for two weeks. Samples treated with 0. 5% PAS and 0.5% AS showed similar folding endurance to the control, but those treated with 5% PAS or 5% AS showed markedly lower endurance. The presence of 1% animal glue had little influence on the result. Other experiments were carried out on the same papers by applying nine different concentrations of PAS (0.5 to 4.5%) and then artificially aging the samples according to the same conditions used previously. Folding endurance, pH value, and brightness were measured before and after two weeks aging. The folding endurance of the aged samples treated with a PAS solution higher than 3.5%, i.e., of pH less than 5, decreased by one half. From the results obtained the authors conclude that paper treated by <i>dosa</i> becomes quickly damaged when the pH of the paper is lower than 5. -- AATA