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).