Due to anthropogenic emissions, sea water pH has decreased about 0.1 units since preindustrial times, and will continue to decline up to 0.4 units by the end of this century. The acclimation capacity of marine life to ocean acidification is a subject of rising interest and importance, still studies on animals with well-developed ion- regulating capacities, such as fish, are comparatively scarce. In this study, acclimation capacities of European sea bass (Dicentrarchus labrax) haemoglobin (Hb) on a functional and molecular level have been researched, by means of oxygen equilibrium curves and qPCR. Fish were reared under three different environmental relevant CO2 partial pressures, with a total incubation time of 33 months. Results show that neither oxygen tension at 50% protein saturation (P50) and cooperativity (n50), nor haematocrit and isoform expression levels change under environmental hypercapnia. The pH dependant O2 affinity (pH50) was found to significantly decrease in both high CO2 treatments. Since molecular analyses suggest that changes in pH sensitivity are not caused by a shift in Hb isoforms to more pH sensitive proteins, an over-compensatory blood alkalosis, followed by chronically elevated erythrocytic organic phosphate levels (GTP and ATP) are assumed. Data indicate that sea bass Hb is adapted to changes in sea water pH, which can be linked to its ecological niche in coastal and estuarine habitats, where fluctuations in abiotic parameters, such as pH, are frequent. Nevertheless, if ocean acidification causes a chronic elevation in organic phosphate levels, sea bass would be facing an energetically costly regulation to maintain respiratory homeostasis, and it would consequently reduce the overall fitness of the animal.