Reliably predicting how coral calcification may respond to ocean acidification and warming depends on our understanding of coral calcification mechanisms. However, the concentration and speciation of dissolved inorganic carbon (DIC) inside corals remain unclear, as only pH has been measured while a necessary second parameter to constrain carbonate chemistry has been missing. Here we report the first carbonate ion concentration ([CO 3 2' ]) measurements together with pH inside corals during the light period. We observe sharp increases in [CO 3 2' ] and pH from the gastric cavity to the calcifying fluid, confirming the existence of a proton (H +) pumping mechanism. We also show that corals can achieve a high aragonite saturation state (Oarag) in the calcifying fluid by elevating pH while at the same time keeping [DIC] low. Such a mechanism may require less H + -pumping and energy for upregulating pH compared with the high [DIC] scenario and thus may allow corals to be more resistant to climate change related stressors.