Abstract: The measuring probe is equipped with a pellistor and renders possible directly determining in a trouble-free manner volatile constituents which are contained in a liquid medium to be investigated and which can be chemically converted with an accompanying heat effect. Heating devices are provided and, due to the prevention of condensate formation, greatly contribute to obtaining high measuring precision. The response time of the measuring probe can also be significantly reduced by heating a membrane which separates a measuring space containing the pellistor from the liquid medium to be investigated. The measuring probe can be used with advantage for monitoring microbiological processes, for example, in a fermenter.