[en] In several nuclear power stations carbon filters in the off-gas system are now used to reduce the emission of radioactive noble gases. A gradual deterioration of the filters can be expected. Methods for measuring the efficiency of filters and the quality of the carbon should therefore be available. A reduction in the separative ability for krypton should be a useful way to indicate an impaired retention for xenon. The advantage of this method lies in the in-situ process which does not affect the operation of the system. An experimental set up and equipment has been tested to measure the dynamic adsorption constant of zenon. Two carbon qualities have been measured at 25degC and - 70degC.(G.B.)