The mechanisms by which organisms control the stability of amorphous calcium carbonate (ACC) are yet not fully understood. Previous studies have shown that the intrinsic properties of ACC and its environment are critical in determining ACC stability. Here, the question, what is the effect of bulk incorporation versus surface adsorption of additives on the stability of synthetic ACC, is addressed. Using a wide range of in situ characterization techniques, it is shown that surface adsorption of poly(Aspartic acid) (pAsp) has a much larger stabilization effect than bulk incorporation of pAsp and only 1.5% pAsp could dramatically increase the crystallization temperature from 141 to 350 °C. On the contrary, surface adsorption of PO43− ions and OH− ions does not effectively stabilize ACC. However, bulk incorporation of these ions could significantly improve the ACC stability. It is concluded that the stabilization mechanism of pAsp is entirely different from that of PO43− and OH− ions: while pAsp is effectively inhibiting calcite nucleation at the surface of ACC particle, the latter acts to modify the ion mobility and delay crystal propagation. Thus, new insights on controlling the stability and crystallization processes of metastable amorphous materials are provided.