Rising levels of atmospheric carbon dioxide could be curbed by large-scale sequestration of CO₂ in the deep sea. Such a solution requires prior assessment of the impact of hypercapnic, acidic seawater on deep-sea fauna. Laboratory studies were conducted to assess the short-term hypercapnic tolerance of the deep-sea Tanner crabChionoecetes tanneri, collected from 1000 m depth in Monterey Canyon off the coast of central California, USA. Hemolymph acid–base parameters were monitored over 24 h of exposure to seawater equilibrated with ~1% CO₂ (seawater$\mathrm{P}_{\mathrm{CO}_{2}}$~6 torr or 0.8 kPa, pH 7.1), and compared with those of the shallow-living Dungeness crabCancer magister. Short-term hypercapnia-induced acidosis in the hemolymph ofChionoecetes tanneriwas almost uncompensated, with a net 24 h pH reduction of 0.32 units and a net bicarbonate accumulation of only 3 mM. Under simultaneous hypercapnia and hypoxia, short-term extracellular acidosis inChionoecetes tanneriwas completely uncompensated. In contrast,Cancer magisterfully recovered its hemolymph pH over 24 h of hypercapnic exposure by net accumulation of 12 mM bicarbonate from the surrounding medium. The data support the hypothesis that deep-sea animals, which are adapted to a stable environment and exhibit reduced metabolic rates, lack the short-term acid–base regulatory capacity to cope with the acute hypercapnic stress that would accompany large-scale CO₂ sequestration. Additionally, the data indicate that sequestration in oxygen-poor areas of the ocean would be even more detrimental to deep-sea fauna.