Using the acid-rich polymeric additives, poly-(L)-aspartate (polyD), poly-(L)-glutamate (polyE), and polyacrylate (polyAA), the structure and morphology of calcium oxalate crystals were controlled during crystallization. In crystallization without the polymeric additives, twinned calcium oxalate monohydrate (COM) crystals were preferentially produced. However, the structural shift of calcium oxalate from a monohydrate (COM) to a dihydrate (COD) form occurred gradually when increasing the polymeric additive concentration, then COD crystals were exclusively crystallized beyond certain critical concentrations of the additives. These critical concentrations actually depended on the molecular configurations of the additives that determined the capability to control the crystallization. When increasing the additive concentration far beyond the critical level, shape modifications without structural changes occurred from a normal octahedral shape (bi-pyramid) to a rod shape (elongated octahedral shape), followed by a dumbbell shape, and finally a sphere shape. Due to its high binding affinity to the crystal surface, polyAA exhibited the most effective elongation in the [0 0 1] direction and clearest development of I 10 01 faces, while polyE had the least effect on the crystals. (c) 2005 Elsevier B.V. All rights reserved.