Manipulation of the crystal-size distribution (CSD) of gypsum from flue gas desulfurization (FGD) liquors was studied using a bench-scale classified crystallizer configuration of the double draw-off (DDO) type. Operation of a DDO crystallizer allows particle separation with increased residence time of product particles producing a significantly larger-sized product. Variables which were studied for their effect on CSD were overflow/underflow ratio, agitator rpm, over-all liquid phase retention time, pH, citric acid (crystal modifier) concentration, rate of make based on precipitant feeds and crystallizer configuration, i. e. DDO or Mixed Suspension Mixed Product Removal (MSMPR). Conditions were found using the DDO configuration with citric acid addition to consistently produce a larger-size gypsum product averaging 130 to 180 mu m (underflow, mass average size) with an equant polycrystalline habit of spherical shape having excellent filtation properties. (Edited author abstract. )