Abstract: A method measures the concentration of the partial pressure of gases, particularly oxygen, in fluids. Know methods for measurements under rough conditions, e.g., measurement of the oxygen partial pressure in grounds, dumps or in the ground water are completely unsuitable. In the present method as described, in the internal area of a hollow vessel closed off to the outside, whose wall consists at least partially of a gas-specific permeable synthetic material that is in contact with the fluid with its external side, the partial pressure change proportional to the gas concentration is measured as a time-scanning of pressure change or as a change of a pressure-dependent physical variable. The measuring principle is very simple and is particularly suitable for the determination and long-time monitoring of the oxygen concentration and of further gases and can also be used in problematic locations.