The effects of Taylor vortices on the mean size, size distribution, and morphology of calcium carbonate crystallized by the reaction of gaseous carbon dioxide and aqueous calcium hydroxide were experimentally investigated in a Couette-Taylor reactor. Via the absorption Of CO2 gas into pure water, the mass transfer coefficient Of CO2 at the gasliquid interface was measured relative to the rotating speed of the inner cylinders in the Couette-Taylor reactor. Polymorphs of rhombohedral-, spindle-, and needle-shaped calcium carbonate crystals were observed with a change in the operating variables, including the reactant concentration and flow rate and the rotating speed of the inner cylinder in the :Couette-Taylor reactor. Using a dimensionless parameter consisting of the operating variables, it was demonstrated that the morphological changes in the calcium carbonate crystals in the present reactor could be anticipated from the operating conditions. In the crystallization, the mean crystal size, predominantly controlled by crystal agglomeration, was most influenced by the fluid dynamic conditions related to the rotating speed of the inner cylinder and reactor geometry. (C) 2003 Elsevier Science B.V. All rights reserved.