The synergistic effects of ocean acidification (OA) and warming were studied on the king scallop (= great scallop; Pecten maximus, L.), an actively swimming calcifier. Metabolic activity and survival success were investigated on the organismic level using oxygen measurements and force recordings during routine metabolism and swimming activity (escape response), respectively. Experiments on P. maximus sampled during winter from Stavanger (Norway) incubated at 4°C and at 10°C for 6-8 weeks at CO2 levels of around 0.039 and 0.112 kPa (390 and 1120 ppm) in re-circulated systems suggest that OA alone has only an marginal impact on routine metabolism and escape response of the scallops. However, we found a significant reduction in both force production and on the quotient of exhausted exercise metabolism to routine metabolism (factorial aerobic scope) in the group incubated under elevated temperature and high CO2 conditions. Hemolymph data revealed, that exhausted animals had significant less oxygen and more CO2 in their hemolymph compared to animals under routine conditions. Scallops incubated at 0.039 kPa had less CO2 in their hemolymph compared to animals at high CO2 conditions. Our data support the hypothesis of Pörtner and Farrell (Science, 2008) that increased CO2 concentrations will effect thermal tolerance of scallops by narrowing the “window” of optimal life conditions.