Calcium sulfate dihydrate primary nucleation was experimentally studied in a batch crystallizer with a related optical device at a supersaturation ranging from 2 to 4 and a fixed temperature T = 25 degreesC. The mother liquor was supersaturated with calcium sulfate in which calcium or sodium chloride salts were added to approach flue gas desulfurization stack-gas liquor. The experimental results were compared with those obtained in the absence of foreign substances in solutions, showing that chloride salts influence gypsum nucleation by increasing the induction period, thus retarding nucleation kinetics.