The present invention relates to a process for determining the state of an exhaust-gas purification device. The exhaust-gas purification device is one which can store gas and/or particles. By means of the proposed process, it is for example possible for the loading state of the exhaust-gas aftertreatment system, for example the oxygen storage state of a catalytic converter which is provided with an oxygen-storing material, such as for example a three-way catalytic converter, to be analysed. The present process operates contactlessly through the analysis of resonances which arise upon the excitation of the catalytic converter using the purification device as microwave cavity resonator