Mosaic, single-crystal CaCO3 thin films have been prepared on modified poly(ethylene terephthalate) (PET) templates. Surface modification of PET through the introduction of carboxylic acid groups (COOH-PET), and the subsequent physical and chemical adsorption of poly(allylamine hydrochloride) (PAH) at pH8 (PAH8-PET) and pH11 (PAH11-PET), afford template surfaces that influenced the phase transition of an amorphous CaCO3 (ACC) films during crystallization in air. Macroscopic ACC thin films are prepared on modified PET films in the presence of poly(acrylic acid). Polycrystalline, spherulitic vaterite (CaCO3) films are observed to form on native PET and PAH11-PET, while mosaic, single-crystal calcitic (CaCO3) films form on COOH-PET and PAH8-PET templates. These results confirm that single-crystal CaCO3 growth patterns are dependent on the surface characteristics of the PET template. We infer therefore, that the nucleation and growth of ceramic films on polymeric templates can be controlled by chemical modification of the polymeric template surface, and by the subsequent attachment of ionic polyelectrolytes.