A method for conditioning air in a test space of a test chamber which receives test material. A temperature in a range of −20° C. to +180° C. is established within the test space with a cooling device. The cooling device includes a cooling circuit with a refrigerant, a heat exchanger, a compressor, a condenser and an expansion element. An internal heat exchanger of the cooling circuit is connected to a high-pressure side of the cooling circuit upstream of the expansion element and downstream of the condenser and to a low-pressure side of the cooling circuit upstream of the compressor and downstream of the heat exchanger and is used to cool the refrigerant of the high-pressure side. A zeotropic refrigerant is used and the internal heat exchanger is used to cool the refrigerant of the high-pressure side to lower an evaporation temperature at the expansion element.