Deterioration of dosa-sized washi was studied for changes in degree of polymerisation (DP), crystallinity and oxidation. DP was calculated from the results of the viscosity measurements of nitrocelluloses, derived from washi samples. The extent of the decrease in cellulose DP of the washi samples was in agreement with their acidity. The DP of the cellulose in the washi fibre, which is higher than that of wood cellulose, may contribute to the permancence of washi. It has been reported that partial recrystallisation of the amorphous region in fibres during ageing decreases the flexibility of paper. By the measurement of X-ray diffraction, kozo, mitsumata and gampi papers showed resistance to recrystallization during the ageing treatment. It may also be surmised that this contributes to the long-term strength resistance of washi. From the measurements of IR spectra, the extent of oxidation reactions in washi seemed to be small. Ageing was at 80 C. And 80% RH for 12 weeks.