A refrigerant for a cooling device (10) comprising a cooling circuit (11) with at least one heat exchanger, in which the refrigerant undergoes a phase transition, the refrigerant being a refrigerant mixture composed of a mass fraction of carbon dioxide and a mass fraction of at least one other component, wherein the mass fraction of carbon dioxide in the refrigerant mixture is 10 to 50 mass percent, preferably 30 to 50 mass percent, the other component being pentafluoroethane and/or difluoromethane.