► We studied the effect of pH and Mg in the crystallization of amorphous CaCO3 (ACC). ► The study combined synchrotron-based scattering with electron microscopy. ► The pH-dependent C speciation and hydration strength of Mg2+ control ACC structure. ► This ACC structure governs the ACC dissolution rate and crystallization pathway. The effects of pH and Mg on the crystallization of amorphous calcium carbonate (ACC) to vaterite and/or calcite were studied using a combination of in situ time resolved synchrotron-based techniques and electron microscopy. The experiments showed that Mg increased the stability of ACC and favoured the formation of calcite over vaterite. A neutral (∼7) starting pH during mixing promoted the transformation of ACC into calcite via a dissolution/reprecipitation mechanism. Conversely, when ACC formed in a solution that started with a high initial pH (∼11.5), the transformation to calcite occurred via metastable vaterite, which formed via a spherulitic growth mechanism. In a second stage this vaterite transformed to calcite via a surface-controlled dissolution and recrystallization mechanism. These crystallization pathways can be explained as a consequence of the pH-dependent composition, local structure, stability and dissolution rates of ACC.