The reactive crystallization kinetics of hydromagnesite (4MgCO(3)center dot Mg(OH)(2)center dot 4H(2)O) for the MgCl2-Na2CO3-NaOH-H2O system has been systematically investigated in a continuously operated mixed-suspension mixed-product removal (MSMPR) crystallizer for the first time. Determination of the effects of reactive temperature and OH- ion on magnesium carbonate hydrates in the above system was conducted through a batch crystallization experiment, and the crystallization temperature of 80 degrees C for the precipitation of regular spherical-like hydromagnesite was selected for the kinetics study. The relative supersaturation for hydromagnesite is obtained based on the activity coefficients calculated by the Pitzer model. The growth rate, nucleation rate, and agglomeration kernel are determined on the basis of the agglomeration population balance equation, and their kinetic equations are then correlated in terms of power law kinetic expressions. The orders of volume growth rate and linear growth rate with respect to the relative supersaturation are 1.55 and 0.95, respectively. The magma density has an important effect on the nucleation rate of hydromagnesite particles. However, the expression of beta alpha M-T(-0.39) for hydromagnesite agglomeration shows that the magma density has a negative effect on the agglomeration kernel. All of these will provide a basis for the design and analysis of industrial crystallizers.