The combination of thermal stress and ocean acidification (OA) can more negatively affect coral calcification than an individual stressors, but the mechanism behind this interaction is unknown. We used two independent methods (microelectrode and boron geochemistry) to measure calcifying fluid pH (pH(cf)) and carbonate chemistry of the corals Pocillopora damicornis and Stylophora pistillata grown under various temperature and pCO(2) conditions. Although these approaches demonstrate that they record pH(cf) over different time scales, they reveal that both species can cope with OA under optimal temperatures (28 degrees C) by elevating pH(cf) and aragonite saturation state (Omega(cf)) in support of calcification. At 31 degrees C, neither species elevated these parameters as they did at 28 degrees C and, likewise, could not maintain substantially positive calcification rates under any pH treatment. These results reveal a previously uncharacterized influence of temperature on coral pH(cf) regulation-the apparent mechanism behind the negative interaction between thermal stress and OA on coral calcification.