We studied the crystallization of CaCO3 by the photoisomerization of azobenzene groups in poly[1-[4-[3-carboxy-4-hydroxyphenylazobenzenesuffonamido]-1,2-ethanediy l, sodium salt] (PAZO) in a mixture of dimethyl sulfoxide and water at 30degreesC. The products were characterized by scanning electron microscopy (SEM), FT-IR, and powder X-ray diffraction (XRD) analysis. We observed that the different shapes of spherical vaterite particles were produced by the changes of configuration and polarity of the azobenzene groups in the polymer which resulted from photo-induced isomerization. The results indicate that the nucleation of primary particles of CaCO3 was inhibited by in situ photoinduced cis-trans isomerization of PAZO. Therefore, we suggest that the shapes of the spherical vaterite can be effectively modified by photoisomerization of the azobenzene groups in the polymer at the initial stage of CaCO3 crystallization. (C) 2004 Elsevier B.V. All rights reserved.