Abstract: The modulation of polyacrylamide (PAM) on the morphology and phase compositions of calcium oxalate crystals was investigated by means of scanning electron microscopy and X-ray powder diffraction. The effects of the concentration of polyacrylamide, pH value in solution and the supersaturation of calcium oxalate on the crystallization of calcium oxalate were discussed. PAM can induce the formation of calcium oxalate dihydrate(COD) crystals and change the morphology of COD and calcium oxalate monohydrate (COM) crystals. In the presence of 5.0g/L of PAM, about 30 percentage (w/w) COD crystals were induced, the aggregated COD crystals were observed, and the shapes of COM crystals were changed from elongated, twinned and aggregated to oval. These results were discussed from the points of molecular structure of PAM, the hydrolysis difference of PAM at various pH values, the interaction between the carboxylic groups of PAM and Ca²⁺
in solution, the complexation-dissociation balance between the PAM and Ca²⁺ ions on the surface of COM crystals, and electrostatic interaction etc. The results obtained show that by changing the experimental conditions calcium oxalate crystals with various phase and morphologies can be prepared.

Key words: polyacrylamide, calcium oxalate, crystallization