A new method is introduced to measure the hydrogen concentration in liquids by applying a polyethersulfone hollow fiber membrane as a probe. Hydrogen rapidly diffuses through the fiber wall after which it is carried to a thermal conductivity detector by an inert gas flow. Stationary measurements in a pure hydrogen atmosphere as well as in a silicone oil are conducted. Polyethersulfone was shown to be more suitable as fiber material for a selective hydrogen probe than polysulfone. From nonstationary measurements it could be concluded that the response time of the polyethersulfone probe is fast enough to monitor the hydrogen concentration during instationary processes which involve hydrogen, like gas-liquid or liquid-solid mass transfer and homogeneous/heterogeneous reactions.