Ocean acidification and possible leakage from subsea CO2 storage may create unfavourable conditions for marine organisms. Deep-living animals are generally believed to be more vulnerable to elevated partial pressure of CO2 (pCO2, environmental hypercapnia) than shallow-living species, but so far only a limited number of studies have investigated the effects of environmental hypercapnia on deep-living animals. In the present study, the deep-water prawn Pandalus borealis (Krøyer, 1838) was subjected to time-dependent exposure to hypercapnic seawater (pCO2 ~ 9100 µatm; pHNBS 6.86) for up to 16 d. Surprisingly, animals were able to partially compensate extracellular acidosis under these severe conditions by accumulating buffering bicarbonate ions at levels comparable to those reported for shallow-living decapod crustaceans. pH regulation was achieved without significantly increasing the ion-regulating activity of total ATPase and Na+/K+-ATPase. Oxygen consumption rate was not substantially affected by exposure. A significant increase in ammonia excretion rate was found within the initial phase of exposure, possibly reflecting H+ buffering by ammonia as a result of increased protein metabolism. The results from this study suggest that the deep-water prawn P. borealis has relatively well-developed mechanisms to counteract CO2-induced acidosis. This finding further indicates greater variation in CO2 tolerance of deep-water organisms than previously assumed and greater tolerance in species with relatively high activity levels compared to less active species. More studies on species from different taxonomic groups and habitat depths are therefore needed to improve the knowledge on how deep-living organisms will respond to the challenges of a high CO2 world.