In the flotation of samples containing gypsum adjusting and maintaining the pH above 8 requires a large amount of base. The pH of the slurry when above 8 decreases when the pulp is aerated as in flotation. To counter, a large amount of lime or other base (for example, sodium carbonate) is consumed. The origin of this effect is the solubility of gypsum. The solubility of gypsum releases Ca2+ ions. As the pH is raised these ions coexist as CaOH+ and Ca(OH)(2)((aq)) which in the presence of air (flotation) are consumed. In this paper a mechanism is proposed to interpret the pH adjustment of a gypsum slurry with and without aeration using lime, soda ash and sodium bicarbonate. Sodium polyphosphate (complexing agent) offers a way to reduce the amount of base needed to raise the pH. (C) 2004 Elsevier Ltd. All rights reserved.