The crystallization kinetics for the simultaneous precipitation of calcium carbonate and magnesium hydroxide were studied. The kinetics for these two precipitations have been studied in a continuous reactor operated under MSMPR (mixed suspension mixed product removal) conditions, first developed by Randolph and Larson('1) for more concentrated systems. The system was studied as a pseudo one component system, with the population balance being applied to this light suspension system ((TURN).2 gm/l). Reactor residence times, effluent alkalinity conditions, and initial iron content (acting as an impurity) were varied to determine these kinetics. The crystal size distribution was measured using the Coulter Counter Model TA II, allowing sizing of the crystals in the 6 to 75 micron size range. The kinetic data were fit with power-law models, indicating nucleation is a nonlinear function of crystal growth. True steady state conditions were not achieved due to the deposition on the walls which increases linearly with time. A "pseudo" steady state condition in which the residual hardness, alkalinity distribution, and crystal size distribution remain fairly constant after approximately 11 residence times, was observed. The MSMPR crystallizer analysis of the population distribution provides an adequate description of the crystal size distribution. The size distributions of the calcium carbonate and magnesium hydroxide components appear to be additive, suggesting the individual precipitations act independently of one another. These components did not appear to act as contaminants of one another;Residual hardness levels were lowest for the lowest T/P conditions used. Crystal habit varies with the effluent supersaturations, with aragonite calcium carbonate crystals found at higher effluent supersaturations (relative supersaturation > 32), and calcite calcium carbonate crystals formed at lower effluent supersaturations (relative supersaturations < 25). The predominant form of the calcium carbonate precipitated in this research was the aragonite form, with a small amount of the calcite formed. The percentage of calcite crystals increased by decreasing the T/P ratio. The brucite form of magnesium hydroxide formed discrete spherical crystals with a flaked structure. The dendritic structure observed for the crystals indicated a large concentration gradient, due to the supersaturation levels involved;('1)Randolph, A. D; Larson, M. A. Theory of particulate processes. New York, NY: Academic Press; 1971. 251 p.