Monohydrocalcite was transformed to aragonite in the presence of Pb in solution to understand the Pb redistribution mechanisms and kinetics during the transformation. Monohydrocalcite (Ca0.98Mg0.02CO3·1.00H2O) was synthesized, and along with Pb stock solution, added to a buffer solution; batch sorption experiments were carried out (i) at different initial-Pb concentrations ranging from 1 to 100μM (1–30 and 50–100μM, referred to as low and high initial-Pb concentrations, respectively) for 24h and (ii) at different reaction time for 1–24h at 1 and 100μM of the initial Pb concentrations. The CO32− concentration (24.4meq/L), pH (9.50) and temperature (25°C) were kept constant within experimental errors throughout the experiments. The transformation of monohydrocalcite was completed in 24h, probably in 15h, at the low initial-Pb concentrations but not even in 24h at the high initial-Pb concentrations, suggesting that the aqueous Pb concentration affects the rates of dissolution of monohydrocalcite and/or nucleation and crystal growth of aragonite, and thus, the transformation rate. Aragonite was shortly formed as monohydrocalcite was dissolved and almost all amount of Ca dissolved from monohydrocalcite formed aragonite. ~90–96% of the initial Pb was adsorbed onto monohydrocalcite before the transformation started, and >95% of the initial Pb was distributed in solid phases at any elapsed time of the reaction and at any initial-Pb concentrations, indicating that the Pb redistribution rate was as fast as the transformation rate of monohydrocalcite. Pb carbonate (possibly hydrocerussite) was formed at the high initial-Pb concentrations but not at the low initial-Pb concentrations. The precipitation rate of the Pb carbonate is slower than the transformation rate of monohydrocalcite. The Pb sorption on aragonite occurred dominantly by incorporation. After the transformation, accordingly, almost all Pb was incorporated in aragonite at the low initial-Pb concentrations, and in aragonite and Pb carbonate at the high initial-Pb concentrations. The incorporation of Pb in aragonite is well explained by the formation of (Ca,Pb)CO3 solid-solution. The Pb redistribution during transformation of monohydrocalcite to aragonite revealed by the present study will give a deeper understanding of trace element transport at the Earth's surface when the transport is accompanied by mineral transformation.