The effect of Mg2+ on the kinetics of calcite crystal growth was investigated using calcite-seeded solutions with a pH-stat system. Five different solution compositions with the same degree of supersaturation with respect to calcite but different CO32-/Ca2+ molar ratios were employed in this study. In the presence of Mg2+, calcite crystal growth rates showed a linear decrease with increasing Mg2+ concentration for the five different solutions employed. By normalizing the crystal growth rate in the presence of Mg2+ to that in the absence of Mg2+, the rate reductions in the five solutions converge to a single line when plotted against the molar ratio of Mg2+/Ca2+. The widely used "empirical degree of supersaturation model" and the simple "Langmuir adsorption model" cannot explain the observed effects of the Mg2+/Ca2+ ratio on the inhibition of calcite crystal growth. Instead, a competitive Langmuir adsorption model involving Ca2+ and Mg2+ was shown to better fit the experimental data. (C) 2009 Elsevier B.V. All rights reserved.