With global climate change ocean warming and acidification occur concomitantly. In this study, we tested the hypothesis that increasing CO2 levels affect the acid-base balance and reduce the activity capacity of the Arctic spider crab Hyas araneus, especially at the limits of thermal tolerance. Crabs were acclimated to projected oceanic CO2 levels for 12 days (today: 380, towards the year 2100: 750 and 1,120, and beyond: 3,000 µatm) and at two temperatures (1° and 4°C). Effects of these treatments on the righting response (RR) were determined 1) at acclimation temperatures followed by 2) righting when exposed to an additional acute (15 min) heat stress at 12°C. Prior to (resting) and after the consecutive stresses of combined righting activity and heat exposure, acid-base status and lactate contents were measured in the haemolymph. Under resting conditions, CO2 caused a decrease in haemolymph pH and an increase in oxygen partial pressure. Despite some buffering via an accumulation of bicarbonate, the extracellular acidosis remained uncompensated at 1°C, a trend exacerbated when animals were acclimated to 4°C. The additional combined exposure to activity and heat had only a slight effect on blood gas and acid-base status. Righting activity in all crabs incubated at 1° and 4°C was unaffected by elevated CO2 levels or acute heat stress but was significantly reduced when both stressors acted synergistically. This impact was much stronger in the group acclimated at 1°C where some individuals acclimated to high CO2 levels stopped responding. Lactate only accumulated in the haemolymph after combined righting and heat stress. In the group acclimated to 1°C lactate content was highest under normocapnia and lowest at the highest CO2 level in line with the finding that RR was largely reduced. In crabs acclimated to 4°C the RR was less affected by CO2 such that activity caused lactate to increase with rising CO2 levels. In line with the concept of oxygen and capacity limited thermal tolerance, all animals exposed to temperature extremes displayed a reduction in scope for performance, a trend exacerbated by increasing CO2 levels. Additionally, the differences seen between cold and warm acclimated Hyas araneus after heat stress indicate that a small shift to higher acclimation temperatures also alleviates the response to temperature extremes, indicating a shift in the thermal tolerance window which reduces susceptibility to additional CO2 exposure.