A test chamber and a method for conditioning air in a temperature-insulated test space of a test chamber, which is sealable against an environment and serves for receiving test material, a temperature ranging from −20° C. to +180° C. being produced within the test space by means of a cooling device of a temperature control device of the test chamber, using a cooling circuit with carbon dioxide (CO2) as a cooling agent, using a heat exchanger in the test space, using a low-pressure compressor and using a high-pressure compressor downstream of the low-pressure compressor, using a gas cooler, using a storage means for the cooling agent and using an expansion valve, the temperature in the test space being controlled and/or regulated by means of a control device of the test chamber. A gaseous and/or liquid cooling agent is dosed in the storage means by means of a high-pressure valve of the cooling circuit downstream of the gas cooler, the storage means being connected to a medium-pressure side of the cooling circuit upstream of the high-pressure compressor and downstream of the low-pressure compressor via a medium-pressure bypass of the cooling circuit, the gaseous cooling agent being dosed in the medium-pressure side from the storage means by means of a medium-pressure valve when the low-pressure compressor is switched off.