Thermal tolerance limits of ectotherms may result from respiratory limitations. In response to declining oxygen availability, organisms have shown to exhibit oxyregulation by enhancing ventilation and heartbeat rates. In this study we examined how this regulatory response in mussels (Mytilus) changes with increasing temperature. Experimental mussels showed extensive oxyregulation at temperatures near to their habitat temperature, but increasingly lost this capacity towards higher temperatures. At breakpoint temperature no regulation took place and respiration rates changed proportional to oxygen availability. These results revealed how thermal limitations relate to respiratory capacity of mussels.