A device for equilibrating small quantities of a liquid reagent or a test solution with a gas-containing liquid or a gas in which at least part of the passage of the reagent leads through a hollow space that is separated by a permeable wall from a gas chamber which is closed, except for an inlet and an outlet opening, and which may be charged with the gas or gas-containing liquid. In order to ensure good mixing of the test liquid during equilibration, the hollow space carrying the liquid is configured as a tube or hose with one or more helical sections inside the gas chamber whose walls are permeable to the gas.