A device for fractionally collecting contents of exhaled air by changing the state of matter of the contents by means of the Joule-Thomson effect arising during the expansion of pressurized gas. The temperature of the exhaled air conducted in a flow channel is lowered, because of the cooling of the expanding gas, to a temperature that is suitable for condensing the contents in the exhaled air flow. This device and method provides a more efficient means by which condensate can be separated from exhaled air in comparison to conveying exhaled air along a cooling surface for the condensation of the contents, as a result of which condensation of the contents occurs only in the area of these cooling surfaces, so that several breath cycles are required for the collection of sufficient condensate for analysis.