<jats:p>The world's oceans are acidifying and warming due to increasing atmospheric CO2 concentrations. Thermal tolerance of fish much depends on the cardiovascular ability to supply the tissues with oxygen. The highly oxygen dependent heart mitochondria thus might play a key role in shaping an organism's tolerance to temperature. The present study aimed to investigate the effects of acute and chronic warming on respiratory capacities of European sea bass (Dicentrarchus labrax L.) heart mitochondria. We hypothesized that acute warming would impair mitochondrial respiratory capacities, but be compensated by life-time conditioning. Increasing PCO2 may additionally cause shifts in metabolic pathways by inhibiting several enzymes of the cellular energy metabolism.</jats:p> <jats:p>Amongst others, acute warming of heart mitochondria of cold-conditioned fish increased leak respiration rates, suggesting a lower aerobic capacity to synthesize ATP with acute warming. However, thermal conditioning increased mitochondrial functionality, e.g. higher respiratory control ratios in heart mitochondria of warm-conditioned compared to cold-conditioned fish. Exposure to high PCO2 synergistically amplified the effects of acute and long-term warming, but did not result in changes by itself. We explained the high ability to maintain mitochondrial function under ocean acidification with the fact that seabass are generally able to acclimate to a variety of environmental conditions. Improved mitochondrial energy metabolism after warm conditioning could be due to the origin of this species in the warm waters of the Mediterranean. Our results also indicate that seabass are not yet fully adapted to the colder temperatures in their northern distribution range and might benefit from warmer temperatures in these latitudes.</jats:p>