In this study, we used the minute gas-liquid interfaces around CO2/NH3 microbubbles as new reaction fields where the crystal nucleation progresses and developed a crystallization technique to control the polymorphism of calcium carbonate (CaCO3). In the regions around the gas-liquid interfaces of CO2/NH3 microbubbles, Ca2+ and CO32- ion concentrations can be adjusted because of the characteristic of the electric charge on the bubble surface and the decrease in CO2 concentration based on unit bubble caused by minimization of bubble size, and because of the pH difference between local pH at the gas-liquid interface and overall pH in the bulk liquid caused by mixing of NH3 with CO2; hence, the polymorph change of CaCO3 is expected to occur. CaCO3 was crystallized at 298 K by a semi-batch type reaction in which CO2/NH3/N-2 bubbles were continuously supplied to an aqueous Ca(NO3)(2) solution using a self-supporting bubble generator. The solution pH during crystallization was maintained at a constant level of 6.9-12.0 by adding HNO3 and NH4OH solution. The average bubble size (d(bbl)) was varied in the range of 40-1000 mu m by controlling the N-2 flow rate, and the molar ratio of CO2/NH3 (alpha(CO2/NH3)) was set at a specified value of 0.20-1.00 at a constant CO2 flow rate. The following results were obtained by varying solution pH, d(bbl), and alpha(CO2/NH3): at a constant d(bbl) of 40 mu m and alpha(CO2/NH3) of 0.20, vaterite and calcite were major products at a solution pH lower than 9.0 and at a solution pH greater than 11.0, respectively, while aragonite was crystallized predominantly in the solution pH range of 9.7-10.5; at a constant solution pH of 9.7 the crystallization of aragonite was accelerated remarkably with a decrease in alpha(CO2/NH3) and d(bbl). (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.