Thin-film growth of aragonite CaCO3 on annealed poly(vinyl alcohol) (PVA) matrices is induced by adding Mg(2+) into a supersaturated solution of CaCO3. Both the growth rate and surface morphology of the aragonite thin films depend upon the concentration of Mg(2+) in the mineralization solution. In the absence of PVA matrices, no thin films are formed, despite the presence of Mg(2+). Molecular dynamics simulation of the CaCO3 precursor suggests that the transition of amorphous calcium carbonate to crystals is suppressed in the presence of Mg(2+). The role for ionic additives in the crystallization of CaCO3 on organic templates obtained in this study may provide useful information for the development of functional hybrid materials.