In order to obtain CaCO3 crystals with high aragonite content in the causticizing reaction in the paper industry, the effect of the NaOH addition at various temperatures was investigated using a batch operation. It is apparent that the NaOH addition increased aragonite composition in the crystal at all temperatures investigated. Then, aragonite crystallization was investigated using a continuous operation. In the continuous operation, it appeared that the dominant factor for the aragonite formation is the low molar ratio of Na2CO3 to Ca(OH)(2). The lower supersaturation and the higher ratio of [Ca2+]/[CO32-] in the presence of NaOH are advantageous to precipitate aragonite in the causticizing reaction. (C) 2003 Elsevier Science B.V. All rights reserved.