Abstract: An method and an apparatus for measuring the partial pressure of gases dissolved in liquids in plants for carrying out biotechnological and food technological processes includes a measuring space which is separated by a gas-permeable diaphragm which is permeable for the gas to be determined. A light emission source produces a light ray with a wavelength which is absorbed by the gas to be determined. The light ray is directed into the measuring space. A measuring device is provided for determining the light ray leaving the measuring space. The measuring space, the light emission and the measuring device are arranged in a rod-shaped probe. The probe is capable of being sterilized. The measuring space is filled with a fluid which does not chemically react with the gas to be determined.