This study investigates the relationship between acidity and the mechanical properties of two Japanese tissue papers (Yukyu-shi and Sekishu Mare), which were treated with a range of plant dyes, watercolours and acrylic paints and then exposed to artificial ageing. Mechanical strength was correlated with the concentration of hydrogen ion (pH) which is an index of the acidic hydrolysis of cellulose in paper structure. In general, the papers treated with plant dyes were more acidic than those treated with watercolours and acrylic paints. Plant-dyed Japanese papers also displayed less folding and tear resistance after ageing and there was a difference in these properties in both of these papers. The untreated Sekishu Mare papers, as well as those treated with watercolours and acrylic paints, exhibited greater tear resistance than the Yukyu-shi papers treated in the same conditions. The Sekishu Mare and Yukyu-shi papers, both untreated and treated with acrylic paints, as well as the Yukyu-shi papers treated with plant dyes, suffered no loss of folding endurance after ageing, which was unexpected considering that the papers were aged for 12 days at 70°C and 65% relative humidity (RH). This study provides paper conservators with a better understanding of toning materials and their impact on the mechanical and chemical stability of papers often chosen for conservation treatments.