Fuel cell system and method of operating a fuel cell system

The fuel cell system operating method involves feeding a process gas (L) to the fuel cell system (2) with the aid of a liquid ring pump (8). Impurities contained in the process gas are taken up by the operating liquid (F) of the liquid ring pump and the loading of the operating liquid with impurities is monitored. The loading of the operating liquid, especially its conductivity, is measured.