We investigated the hemolymph acid–base status of the black-lip pearl oyster Pinctada margaritifera exposed to air at 23℃. Air-exposed P. margaritifera showed a decrease in hemolymph pH from 7.563 to 6.965 after 36 h. The hemolymph total CO_2 concentration (Tco_2) increased from 2.01 mM/L to 5.63 mM/L during 36 h of air exposure. The hemolymph CO_2 partial pressure (Pco_2) and bicarbonate concentration ([HCO_3^–]) were calculated, by rearranging the Henderson–Hasselbalch equation with substitution of CO_2 solubility coefficient (αco_2, 40.49 mM/L/torr) and apparent dissociation constant of carbonic acid (pKapp, 5.98351). The hemolymph Pco_2 increased from 1.28 torr to 13.68 torr, and [HCO_3^–] increased from 1.96 mM/L to 5.08 mM/L during 36 h of air exposure. The hemolymph calcium concentration ([Ca^{2+}]) increased from 7.96 mM/L to 11.2 mM/L. These results indicated that during prolonged air exposure P. margaritifera showed hemolymph acidosis with partial metabolic compensation by the mobilized bicarbonate from the shell valve. When the experimental animals were immersed in seawater after air exposure, P. margaritifera needed 24 hours at least for recovery of the hemolymph acid–base status to the initial level.