Using recently-developed instrumentation for the continuous measurement and control of pH under cooking conditions, a series of sodium base cooks was made in which pH at cooking temperature was maintained at the levels 2.80, 3.00, and 3.20 by injection of mineral acids and bases. In the range studied, time at temperature to constant yield was found to be a linear function of pH. HCl gave anomalous results, requiring longer time to fixed yield. Despite wide variations in liquor composition and digester pressure, the pulps were of similar quality, and only viscosity showed a certain response to the strength of the acids used. The quantities of strong acid or base required for equal pH adjustments from a reference level gave a measure of the buffer capacity of the digester contents. Continuous control of acidity at a fixed temperature and pH level by means of sodium hydroxide solution of known strength indicated the amount of acid formed during a typical cook. Recent publications on sulphite pulping and some aspects of pH control in industrial practice are discussed in the light of the present findings.