Batch cooling crystallization of potassium alum was performed in the presence of an azo dye, Bismarck Brown G, as a crystal-growth suppressor. The crystal size of the product was caused to increase unexpectedly by the presence of the growth suppressor. This is closely related to the decrease in the number of crystals in the crystallizer and the resultant increase in supersaturation. Such a suppressor effect is pointed out to be important from an industrial point of view.