Crystallization of a CaCO3 thin film from supersaturated solution on chitosan in the presence of poly-acrylic acid was investigated by in situ AFM and SNOM. It was directly observed that crystallization proceeds through characteristic stages consistent with the theory of spherulitic growth: development of individual branches, build-up of larger scale "lobe" features, followed by overgrowth and ripening. We propose that crystallization of CaCO3 on chitosan in the presence of poly(acrylic acid) (PAA) proceeds in a gelatinous matrix formed by PAA stabilized amorphous CaCO3,. the spherulitic character of crystallization is initiated by the high viscosity of gel and presence of PAA impurities. Characteristic sizes of spherulite features deviate significantly from the prediction based on diffusivity of PAA in water due to low diffusivity of PAA in gel. (c) 2007 Elsevier B.V. All rights reserved.