Instruments and methods of pH measurements are described. The decay of cellulose by acid is explained, and the most important causes of acidity and acid decay of paper are given, as well as some preventive measures. An extensive list of references follows the paper. In part II, the effects of conservation and restoration measures on pH-values are discussed. Deacidification, alkaline reserve and buffering are described. Changes in pH-values and alkaline reserve induced by bleaching, watering and leafcasting are described and explained. -- AATA