AbstractA quantitative estimation of the effect of sodium hydroxide on the solubility of gypsum in the presence of lime at 30, 60 and 100°C is given. At 30°C the solubility of gypsum was promoted by a 0.0146M solution of sodium hydroxide which at a concentration as low as 0.0029M raised the sulphate ion concentration at 60 and 100°C. At the three temperatures studied the solubility curves of gypsum first rose slowly then more rapidly around 0.02 to 0.08M NaOH. In the range 0.08 to 1M NaOH solution, a constant value of the sulphate ion concentration was measured which corresponded to the complete dissolution of gypsum, even in the presence of lime. The solubility curve of gypsum was lowered slightly at 30°C in a supersaturated lime solution and was depressed by concentrations lower than or equal to 0.02M NaOH solutions at 60 and 10