The phase transformation and crystal morphology of calcium oxalate (CaOv) formed in water and sugar solutions and CaOx formed in CaOx-silica (SiO2) systems with and without sugar have been investigated using a combination of instrumental techniques including X-ray powder diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, and FT-IR spectroscopy. Addition of SiO2 or sugar in CaOx crystallization system selectively promoted the formation of COT and COD, respectively, and modified the crystal size and habit of COM crystals, the extent of which was dependant on the concentrations of SiO2 and sugar. CaOx-SiO2 co-precipitation in sugar solutions led to the stabilization of either COD or COT, depending on the initial concentration of CaOx. A variety of mechanisms including electrostatic effects, geometrical matching and stereochemical compatibility between CaOx crystallites and polymeric SiO2/sucrose molecules have been proposed to account for the observed crystallographic and morphological changes of CaOx. (C) 2004 Elsevier B.V. All rights reserved.