With global climate change ocean warming and acidification occur concomitantly. The survival and distribution of species will depend on their existing ability to exploit their physiological plasticity. pH regulation and the energetically costly processes involved appear crucial to sustain the performance of marine organisms and shaping their sensitivity to ocean acidification. Especially under rising temperatures, small pH disturbances in body fluids might already exert critical impact on physiological processes. We therefore studied the effects of ocean acidification on thermal tolerance and acid-base regulation capacity of a sub-Arctic population of Mytilus edulis. Mussels were acclimated for 4 weeks at 10°C and incubated under normocapnia (390µatm) and hypercapnia (1120μatm) for further 2 weeks, before being subjected to an acute warming protocol (10-28°C; 3°C/night). The temperature induced increase in oxygen consumption led to Q10-values of 2.4 (normocapnia) and 2.2 (hypercapnia) indicating a mild limitation of aerobic metabolism in hypercapnic animals. Anaerobic metabolites accumulated above 25°C indicating an upper critical temperature independent of CO2 albeit anaerobic metabolism was lower under hypercapnia. The decrease in haemolymph pH during warming followed the alphastat pattern, with animals under hypercapnia consistently displaying an acidosis when compared to controls. Mantle intracellular pH was initially maintained during warming before a sudden acidification set in. The intracellular acidosis occurred earlier under hypercapnia indicating reduced energy allocation to intracellular pH regulation. The earlier reduction of energy-dependent processes under combined hypercapnia and warming may enhance passive tolerance to temperature extremes in this intertidal specie. However, permanent hypercapnia may only be sustained at the expense of organismic performance, especially at the limits of the thermal performance window.