Ocean acidification threatens the persistence of biogenic calcium carbonate (CaCO 3) production on coral reefs. However, some coral genera show resistance to declines in seawater pH, potentially achieved by modulating the chemistry of the fluid where calcification occurs. We use two novel geochem-ical techniques based on boron systematics and Raman spectroscopy, which together provide the first constraints on the sensitivity of coral calcifying fluid calcium concentrations (½Ca 2þ cf) to changing seawater pH. In response to simulated end-of-century pH conditions, Pocillopora damicornis increased ½Ca 2þ cf to as much as 25% above that of seawater and maintained constant calcification rates. Conversely, Acropora youngei displayed less control over ½Ca 2þ cf , and its calcification rates strongly declined at lower seawater pH. Although the role of ½Ca 2þ cf in driving calcification has often been neglected, increasing ½Ca 2þ cf may be a key mechanism enabling more resistant corals to cope with ocean acidification and continue to build CaCO 3 skeletons in a high-CO 2 world.