The invention relates to a method and device for determining the redox state of an anode (11), which is coated with a catalyst material or consisting thereof, associated with a high temperature fuel cell (10) or a reaction surface (16) of a reformer (13). According to the invention, at least one first resonator (3) of a piezoelectric sensor device (1) is arranged in the anode gas flow (5) of the high temperature fuel cell or in the gas flow of the reformer (13). The first resonator (3) is provided with a coating (4) which is oxidizable or reducible in the gas flow. The device also comprises a device (8) for measuring at least one modification of the resonance behaviour of the first resonator (3). The detected measuring value acts as an indicator of the redox state of the anode (11) of the high temperature fuel cell (10) or reaction surface (16) of the reformer (13).