Abstract: A refrigeration system including one or more first compressor(s) for compressing a carbon dioxide (CO2) refrigerant, a main heat rejection system for cooling the CO2 refrigerant, one or more high pressure expansion device(s) for reducing the pressure of the CO2 refrigerant, a receiver for storing the CO2 refrigerant, one or more high pressure expansion device(s), an evaporator and a receiver pressure regulating device. The refrigeration system further includes an auxiliary refrigeration system including an auxiliary compressor arranged to compress at least part of the CO2 refrigerant and thereafter to direct the compressed CO2 refrigerant to a heat rejection system.