Middle-phase microemulsions (MPMs) in two systems of a cationic surfactant, tetradecyltrimethylammonium bromide (TTABr)/n-butanol/iso-octane/Na2CO3 or CaCl2 and an anionic surfactant, sodium dodecyl sulfate (SDS)/n-butanol/iso-octane/Na2CO3 or CaCl2, were used to synthesize nanostructured calcium carbonates. MPMs provide a simple and versatile reaction media, i.e., upper-phase W/O, BC, and O/W structured equilibrium microemulsions to be used for synthesizing hierarchically structured CaCO3 at the nanometer scale. On the basis of the investigations on the phase behavior of the MPMs, hierarchically structured calcium carbonates with dendrites, ellipsoids, square-schistose cubes, and spheres were synthesized through the MPM-based routes. (c) 2006 Published by Elsevier Inc.